The point here is that self-hosting is _possible_. Most users are almost certainly going to just let a third party host their keys. But if keys are identity primitive at the protocol level, then the protocol prevents full centralized platform capture, similar to email interop.
The key being the identity still has the problem of not being changeable, no matter how you spin it.
Self-hosting is possible with ActivityPub as well, but without sacrificing the user-friendliness. I can already see someone saying "what do you mean I have to start a new account after my private key leaked".
