This can work. I'll even give a couple examples of boss fights that pull off this kind of pacing and feel very satisfying. But it's difficult to pull off, and there are a few things you have to get right for an unusual pacing trick like this to land successfully - especially at the final climax of your story.
A video game is a unique story-telling medium specifically because, in addition to all of the narrative elements that are present in all other storytelling mediums, video games incorporate ludology - that is, the gameplay What this means is that for a video game in particular, if you want an emotionally powerful moment, it's not enough for the story to reach a high point. You must also simultaneously reach a high point in your ludology. This means either creating a spike in difficulty, pushing your player outside of their comfort zone, or throwing a twist into the game's mechanics, forcing your player to quickly adapt to new situations. So in the rest of my answer, I'm going to talk about how to get your story to a high point, how to get your gameplay to a high point, and how it's possible to achieve both even with an anticlimax ending.
Narrtive High Points
In a well-written story, emotional weight primarily comes from four elements:
- Character arcs
- Plot momentum
A scene in your story that ties tightly into one of your story's main themes, shows a character changing or making a truly difficult choice, advances or complicates a major conflict, and propels the plot towards its climax with the stakes continually raising will resonate emotionally with readers.
What's important to notice - very very important to notice - is what is not on the list. These elements do not determine whether a moment in a story is an emotional high point or not:
- The intensity of the action
- The impressiveness of the visuals
- The amount of violence, power, or magic being used
These elements do not weaken a scene, but they don't necessarily strengthen it, either.
Sonic Forces handles this badly
For example, Sonic Forces has, frankly, an absolutely atrocious story. In its climax, we see Sonic and the avatar character leading a huge army against Eggman's fortress. Eggman uses reality-bending magic, there are multiple boss fights against your rival and giant robots, there are huge armies kicking the snot out of each other in the background... The imagery is there. This is a high-intensity climax. But it falls very, very flat.
This is because the elements that give a scene emotional weight are absent:
- The only narrative theme in Sonic Forces is the avatar character becoming cool enough to hang out with Sonic, and that is too thin and fantasy-fulfilling to give the ending any gravity.
- Besides the avatar character, none of the other characters in the game have any character development whatsoever, and we certainly don't see any one making agonizing choices in the climax.
- The conflict between Eggman and Sonic is a very plain, vanilla take on the good guys vs. someone who wants to end the world, so it doesn't pull any weight.
- From the moment you first turn on the game, you already know that the plot ends with Sonic winning and Eggman losing. The climax has a couple of twists, but never enough to make you question where the plot's momentum is going.
The Beginner's Guide handles this well
In contrast, The Beginner's Guide, a walking simulator by the developers of The Stanley Parable, has understated imagery during its climax but is very emotionally powerful. The story is about a man, Wreden, who used to be friends with a game developer, Coda. Wreden hasn't had contact with Coda in several years, so (in the story's universe) he releases a compilation of Coda's games with commentary in an attempt to reconnect with his friend. As you play through Coda's games, they slowly become more and more unhinged, strongly suggesting the Coda had some serious personal demons and tried to use their hobby as a way of working through their pain. Through his commentary, Wreden explains what he tried to do to help Coda deal with these issues, but it slowly becomes clear that Wreden has some demons of his own that he's blind to.
The final scene takes place in a building with some pretty twisted, prison-like architecture and moments where Coda's game design actively tries to make it impossible for the player to finish. This is reasonably severe imagery, but it's nowhere as intense as, for example, Sonic Force's giant robots and explosive magic. This leads up to a moment where the imagery gives way to very comfortable, warm, home-like architecture where Coda has written one final message to Wreden before cutting off contact for good. This change in architecture means that whatever energy the imagery has is completely undercut. The climax takes place in probably the single most inviting and calm environment in the entire story. But it still packs an enormous punch because all of the narrative elements are present:
- There are several themes in the story, and they all come to a head at once. The consequences of Coda and Wreden's personal demons come out in full force. And the ways that Wreden has been trying to help Coda are called out for the ways in which they're toxic and counterproductive.
- This is Coda's final message to Wreden. The fact that Coda is willing to permently end a relationship with a friend they've had for years is a very severe and personal decision for a character to make.
- What Coda specifically says addresses the conflicts that have appeared in a very unusual way.
- One of the first things the game tells you is that Coda and Wreden haven't been in touch for years. This foreshadowing gives the entire plot strong momentum towards learning why this is the case. The climax answers that question, allowing the plot's momentum to come to an end with satisfying gravity.
(I do want to emphasize, though, that strong imagery does not make a bad climax. It's just unrelated to the climax. Stories like Undertale, Bastion, The Lord of the Rings, The Incredibles, and The Hitchhiker's Guide to the Galaxy have bombastic endings that land incredibly well.)
How this applies to your question
What you're suggesting is a final climax with low-intesity imagery. This absolutely can work. What you need to do, though, is figure out how your ending ties into your narrative elements.
For this to work, you have to do two things:
- The difficult boss fight before the final climax can't answer all of your narrative's questions.
- The pushover boss fight at the true climax has to answer the leftover questions.
Right now, from what you've described, it's not clear to me that you have any questions like that. The heros go about there lives for a decade or two, then get back together and punch out the bad guy. What theme does this address? What difficult decisions do the main characters need to make as a result of this, and how are they changed as a result? What conflicts are being addressed, besides the almost flavorless conflict of the big bad being a potential threat to the world? And how is the plot structured so that ending the story at this moment is the most satisfying point, as opposed to the grander boss fight shortly beforehand?
That paragraph came across as snarky, but that's not my intent. You only had a couple of paragraphs to summarize your thoughts on your ending, so of course you didn't give me all of the details I'm asking about! I would not be surprised to learn you already have answers to all of those questions. And even if you don't, the scenario you're considering certainly has room for answers to those questions. Your heros have settled down and have families and lives they want to return to, give you a rich vein to probe for agonizing character choices and thematic material. Your primary antagonist both regrets his past decisions and wants to die. These are both very unusual things for a villian to decide, so you've got a lot of juicy material to explore there. And with a little planning, you can tie the themes that you emphasize during this final, sad encounter throughout the rest of your story, setting up your plot so that it's only satisfyingly resolved when the themes are addressed in this final scene.
Ludological High Points
At its core, gameplay is about solving interesting problems by making complex decisions. When you make the right decisions, you win and progress to the next stage in the game's progression. When you make the wrong decisions, you lose and go back to a checkpoint to try again. The nature of these problems and decisions certainly change from game to game. A JRPG that plays like an Excel spreadsheet with anime clipart everywhere makes the nature of making decisions to solve problems very explicit (and that's not a bad thing at all!). A flashy beat-em-up like God of War or Devil May Cry might seem on the surface to be about action instead of decision making - but your moveset is specifically designed so that different actions have different strengths and weaknesses, so choosing which kind of fighting style to use and which abilities to use when boils down to making rather complex decisions very quickly.
What these means is that there are two ways to make a ludological climax in a video game:
- Use the mechanics you've already established in the game and spike the difficulty up. This tests the player on whether they deeply understand how to make the right decisions for the situation they're in.
- Add in a new mechanic or throw a twist onto an existing mechanic. This forces the player to scramble as they make decisions for new, uncertain situations.
This gives rise to the most common gameplay progression we see in video games. A level or chapter in a game starts out by bumping up the difficulty slightly in order to keep up with the lessons the player learned in the previous chapter, but levels rarely start out particularly challenging. At some point, a new mechanic or twist is introduced, and the rest of the chapter focuses on exploring the implications of the new mechanic. This increases the difficult pretty organically and keeps things interesting. The difficulty continues to gradually increase until you reach the level boss, which spikes the difficulty up dramatically and pushes the new mechanic to its logical conclusion. Then, the cycle repeats in the next chapter, which incorporates the new mechanic for good, drops the intensity of gameplay down from the high of the boss fight, and starts introducing new mechanics once more.
By the time you get to the final boss, you've established all of the mechanics you want to use in your game, so you push all or most of them as far as they'll go all at once while pushing the difficulty to its highest point in the game. Many final bosses also throw in last-minute twists that have obvious implications but nevertheless force the player to think on their toes.
Paper Mario: The Thousand-Year Door is a great example
Paper Mario is a fairly standard turn-based RPG (in terms of its game design) with an excellent final boss. She is a demon queen with more health and damage output than any other mandatory enemy in the game. The nature of the turn-based battle system is about winning by attrition before your enemies can grind you out of all of your resources to put up a viable fight. So the boss's high HP forces you to very carefully manage your pool of mana and HP to last for a long time, and her high damage output puts a lot of pressure on those resources.
In addition, she has several nasty tricks up her sleeve. For the first several turns, she's completely invincible. This invincibility is automatically removed after a scripted cutscene, but this isn't clear the first time you fight her, so you're likely going to be baited into expending a lot of resouces figuring out how to remove her invincibility. This leaves you in a weak position for the rest of the fight. Later, without warning, she kills helpful NPCs that allow you to regenerate one of your mana pools, putting an unexpected and painful restriction on one of your resources partway through the fight. And some of her attacks require you to use very specific abilities to counter them, forcing you to be prepared to either tank lots of damage or switch your strategy quickly to answer her attack cues.
Altogether, this boss fight tests the player on whether they deeply understand the game's core mechanics and also throws them multiple curveballs that can force them to completely rework their strategy on the spot. This makes for a very memorable and satisfying boss fight.
It helps that Paper Mario: TTYD has a reasonably strong story to go along with it, as well! The Shadow Queen is simultaneously the climax of the gameplay and the climax of the story. This makes here the perfect way to end the game.
What this means for your situation
For your anitclimax, you're not likely going to be able to put together a battle that pushes the player to their limits. So instead, you're going to want to throw them a bizzare twist. Even if the battle is ultimately a foregone conclusion, changing the rules at the last second will leave your players thinking very carefully about how they approach the scenario. This will make the anticlimax boss fight still have memorable and satisfying gameplay.
For example, you could set up the final fight so that the boss never attacks you, but you have to solve a puzzle to destroy the artifacts keeping him alive. Or the particular way you kill him determines which ending the player gets - a kind, swift, and magically pure kill gets a good ending, while using dark magic to make the villain suffer gets a bad ending. Or the boss heals dramatically each turn (even though he doesn't want to), forcing the player to crank out as much damage per turn/second as possible to finally bring things to an end. These are just a few ideas. I'm sure that, as someone much closer to your game, you can come up with an idea that fits your game particularly well.
An example of a successful anticlimax final boss
Mario & Luigi: Partners in Time has an excellent example of an anti-climax boss. The high-point final boss is a four-stage battle against two alien princesses. It's an over-the-top marathon that can easily last half an hour to an hour! The game is another turn-based RPG, and the core mechanic is that by putting in the right action commands, the player can completely dodge enemy attacks or even turn them back onto the enemies. In order to survive for long enough to see this marathon final boss to the end, the player must dodge the large majority of the bosses' attacks.
After all of this is over, though, there are still some unresolved questions. Bowser has popped in and out of the story, and his arc hasn't been brought to a satisfying close yet, so the game throws in one last battle to tie up the story (Youtube recording of the fight).
In this battle, Bowser is possessed by one of the alien princesses' spirits and, high on the power she gives him, decides to take out Mario and co. once and for all. This fight only last a couple of minutes and, at this point in the game, is comfortably easy for the player to complete. This is in stark contrast to the neverending final test the player has just finished! So, to make this anticlimax fight feel satisfying, the game throws in a very unexpected twist: The player doesn't get to have any turns. Instead, Bowser constantly lobs fireballs at the player, who can only counterattack using the action command system. This twist forces the player to abandon much of what they learned about the game's give-and-take strategy, knowing when to dish out damage and when to play defensively, and instead hastily adopt a new strategy based on going all-in with counterattacks. This doesn't make the fight much more difficult, but it certainly makes it memorable.