unless the domain is taken away from you. Or the TLD is retired. Or the registrar shuts down and the domain expires and you can't renew it because it's stuck in limbo.
Assuming an old-school TLD (com/net/org/iso 3166 country) that's not run by muppets, any of these things happens less often than hosting providers going under and a lot less often than fediverse hosts throwing in the towel.
I'm not saying it's a panacea nor is it available to everyone, but it's still a shitload easier than self-hosting your fediverse instance. "It's ok to lose your fediverse presence and have to start again because it's also possible to lose your domain name" doesn't strike me as a particularly great argument given the relative probability of those events.
And it doesn't even seem like it'd be especially hard to fix, if the software folk wanted to. A tool to export the database as plain text (JSON, whatever) and another tool to import it in the new place.