That’s a de-facto standard, so it’s safe to say that, given that all major AP implementations do so, your identity is tied to your homeserver.

The tyranny of the installed base is real. What you choose to ship basically defines what everyone else can do with AP in practice.

I think it’s a shame that Mastodon got a million fuzzy blinky UI features before the (still missing) BYOdomain support, given that everyone in that space links permanent identity to user+domain.

I don’t see that changing. Do you? It seems everyone has settled on this, just like email, and that the solution is to just use a domain that you control for your identity (just like email).

"Fuzzy blinky UI features" is what people actually care about though. You can have all the theoretical bells and whistles in the protocol but if you don't have a flagship application that people can use for their everyday needs, nobody is going to care. The fact of the matter is that "porting your account" is just a much less frequent need than literally anything else that people will come across in their day to day use, and the fact that you can move to another account in the fediverse without losing your followers is good enough for most. Of course it would be nice if the old and new accounts were verbatim, not even identifiably different copies, but we're talking about synchronizing (potentially, and likely on average) multiple gigabytes of data across small hobbyist servers that also still have to serve requests from other users. Not to mention the abuseability of being able to import gigabytes of pre-recorded content like that, a spammer's boon. Worth mentioning that the account portability approach described in ATP is just "upload your backup to the other server" which in practice is going to suffer from the exact problems I am describing.