I went through the evolution of email.. At first it was universities, then ISPs etc. Having your identity tied to them SUCKED every time you no longer qualified for an account, changed providers ETC. I was a hotmail user before Microsoft purchased it, and an early beta Gmail user.. While this is some centralisation these itentied have lasted decades, where AT THE TIME AOL was the (this is the biggest, never going away) option. Now almost no one has an @AOL.com address.
Point being that no matter the current promise your instance could DIE if you get ill or can't afford to host it etc. The model is BAD. I have said it before and will say it again, Identity SHOULD NOT be tied to instances, AND it needs some form of bot and trust system built in.