So I'm writing an urban fantasy story about the modern day after a mysterious unknown entity suddenly imbues humans with the ability to use magic powers. While the story initially starts out as a more adult Harry Potter-esque mix of magical slice of life and mystery involving the teenage protagonists, as the story progresses the situation is going to gradually deteriorate as it becomes more and more obvious that life as we know it was not designed to handle humans having magic powers, until finally, in what appears to be a maliciously calculated move by the unknown empowering entity, humanity is given access to powers that completely destroy our modern infrastructure and render nearly all technology useless in the presence of anyone who wants it to be useless.
The result is that from the third act of the first book onward, the setting turns into a sort of post-apocalyptic "power punk" world where large-scale infrastructure is basically rendered impossible for the foreseeable future and humans need to supplement a tech level somewhere around your average zombie apocalypse with supernatural powers they get access to in order to survive in far more isolated and smaller communities that are largely self-sufficient by necessity.
The problem, obviously, is that the average modern person doesn't have that many skills that would be relevant to surviving in a world without the internet, power plants, plumbing networks delivering running water, and the massive supply network producing and delivering food, tools and other goods. Obviously if the story's cast didn't include such people then it would be a very short and unsatisfying story once this went down, but having tons of people with extremely rare skills readily at hand in the cast or swiftly introduced to it when needed sounds like it runs the risk of being incredibly contrived.
One of the biggest complaints I've seen about another book that had characters put in a similar situation, Dies the Fire, a book about a mysterious event permanently rendering all of modern technology useless, was that the two groups the story focused on just seemed to have people with the necessary survival skills just drop into their laps, or otherwise run into ridiculous amounts of luck at the story's start. Now obviously when you're put in a world where the physical laws have been intentionally altered to render guns and electricity and combustion engines useless, the main characters aren't going to survive unless they have people who can hunt, grow crops, make and use medieval-style weapons, build shelter, and treat illnesses without relying on modern technology. But while I mostly enjoyed that book, I can't help but agree that it was a bit ridiculous how people with these absurdly rare skills just seemed to fall into the main characters' laps in rapid succession all throughout the first half of the book.
Of course I've got magic at my disposal too, and so I can choose to address some issues with characters' skills and others with magical abilities, but that also seems to run the risk of being contrived if I don't balance it properly and work out how much luck is necessary for survival and don't accidentally sacrifice dramatic tension by getting rid of problems it would be far more interesting for them to have but still believable for them to solve.
How can I work out what I can and can't get away with in terms of populating my cast with characters with rare but survival-necessary skills, and which problems I should solve with luck and which should be solved with skill and hardship on the part of the main cast?