Yeah, I wasn't arguing, just thinking out loud too. I think the whole decentralised aspect of the fediverse means that ownership has to have a cryptographic answer because there's no central source of truth that everyone can agree on.
I think moving accounts is a little easier than you think, apart from who gets to say that something should move. It'd be better to have a "pull" than something like the "push" solution that currently exists on Mastodon - there you can forward an account to a new place, as long as the old instance exists and cooperates (big ifs).
I'm mostly thinking about moving accounts (+ communities) in the case of when an instance suddenly vanishes.