I don't think you need to fear compatibility issues moving forward with Word, as it's more or less the business world's linga franca of text document file formats. And even if it goes belly-up, there's so much information corporations rely on that is captured in it, there will be conversion utilities.
What you need to ask yourself is what's missing in your toolchain? What features do you need to help you write better with your specific working processes and methods? Do they cover your entire workflow? Do you need a one-stop shop? Or do you prefer multiple applications in your toolchain? What do you want to use to write, edit, collaborate, and deliver your document? If Word does everything you want, then stick with Word. Just because there are newer toys out there doesn't necessarily mean they're better toys for what you want to get done.
Word is a pretty dang good word processor. But if you need to lay out documents in multiple columns for magazine, newsletter, or newspaper layout, it can suck royally. If you tend to write in a At that point, a page layout application might make more sense. It also sucks as a content management system (CMS). It sucks as an ebook authoring tool. It sucks for really long complex documents. (I prefer FrameMaker, but I'm a dinosaur with vi and nroff/troff skillz, and can still remember when Frame was cross-platform and produced by Frame not Adobe. :)
Scrivener isn't a word processor so much as it's an IDE for document development. It's a different kind of tool. If you're the kind of person who works with a cluttered desk, keeps multiple notes in multiple formats (text, graphics, links), and likes to have the ability to glance over all your research or ideas at once on a big board, or to be kept tidy in whatever organization you like, and to be able to refer to that material while writing, then Scrivener can help do all of that on the computer. It can foster writing in non-linear fashion. Word, not so much.
But. Keep in mind that when getting a book published, chances are good that for collaborating on edits with a copy editor and editor, chances are good you'll have to go back to Word because of the business world lingua franca thing. See Charles Stross's blog entry on "Writing a novel in Scrivener: lessons learned".