Eh, tbh though, I don't know why I was arguing that point, it really doesn't matter one way or another.
You absolutely identified a very good reason to want to switch instances, and why account portability has value in the Bluesky world.
I wish ActivityPub had a better solve for this, but unfortunately a true federated model makes that awfully difficult (since the entire ecosystem has to update its view of where posts are located).
Similarly, if I'm in a length email exchange with a friend on Gmail, and then I move to another service, there's no way to update my friend's account to "rewrite" those emails to point to the new service. It's a deep limitation in the model itself. The only question is whether the tradeoff is worth it (and, to me, it is).
(you can also change which domain your bsky profile is identified by, which, I'm not quite sure how that works at a protocol level but it is possible)
Until you lose control of the domain for any number of reasons and then whoever buys it happily continue on as you. A simple credit card processing problem can lose your "identity" at some point in the future.
Also, to emphasize, regular people who don't do those other things don't have to roll a custom domain on bsky (and are unlikely to). But eg. if NPR wants to make a bsky account under @npr.org, that's really quite good verification that it's "the real NPR account", and nobody has to deal with any of the politics around verification which have become a dumpster-fire on twitter since Musk took over
A number of such events have occurred.
I haven't actually heard of anyone moderated off an instance (aside from spammers), but maybe I hang with the "wrong" crowd for that.