So, I think Bluesky has it wrong and the Mastodon model is still better (theoretically, of course, I have no idea how to make this more popular) ; which is to say -- there are no huge advantages and probably tons of pitfalls when you try to do "one universal identifier that sticks forever" for people.

We already have a better working model for this; it's HOW WE DO EMAIL. On one hand, email is a very strong "centralized identity" thing, and on the other it's not absolute. Yes, it has problems, but as twitter has shown -- deeply centralizing the thing has MORE problems.