It’d be neat if the community itself could vote to migrate to a new instance
You kind of already can do this. It’s just that instead of voting directly, people choose individually where to go instead. That is also kind of a “vote” - you vote by choosing a community and so whichever gets most votes becomes the new major community of that topic.
There is no need for a systematic solution, it is already in place. The admins/mods of lemmy.ml are acting in questionable ways and people are pointing this out and some are even trying to rally to defederate and trying to get people to move off the instance and all that. This is the systematic solution. The system is working as intended.