At least there's already a relatively established dispute mechanism for domains.
This wouldn't be a big deal in practice (besides losing the domain). Domain usernames are just the combo of you telling Bluesky "I intend to use this domain name" and then you placing a TXT record on the domain to prove you own it. If you want to change domains (or, are forced to), you just give them the new domain name and you set another TXT record (just like if you had set up a domain name as a username for the first time). The underlying DID is still yours.
https://news.ycombinator.com/item?id=11349870 - Kik, left-pad, and npm (2016)
The identity is just a public key, basically. There's a resolution service which uses domains, but you can move the identity, with all the data, to another one.
Granted, my comment doesn’t add much to the discussion, since this domain ownership issue would have been a problem in ActivityPub too.