From what I've seen, communities don't migrate; they fragment. It's very hard to get a group to move, even if the new option is clearly better in the ways that matter to the group.

When I left Stack Exchange I joined the Codidact project, which is FLOSS and run by a non-profit foundation (disclosure: I'm on the board). We still have lots of things we want to improve (we're a very small team), but probably our biggest challenge is adoption -- attracting enough people who want to do Q&A and related knowledge-sharing somewhere that puts communities and people first and isn't driven by revenue goals. We've got communities for software development, Linux, and others, and I'd love to see them grow.