If you're moving around between servers it means that each server has to validate the users. The matrix solution was to add identity servers that tell the server you're logging into who you are. I imagine something similar would be necessary to hop servers on nostr and bluesky. Its possible the connected servers share user identities but that has its own array of problems.

Then users need to spin up their own instance, right? Then all their posts stay backed up if they choose?

Seems like a mediocre problem to get bent out of shape enough to move to a centralized platform.