Celebrities will never adopt the Fediverse until usernames are centralized.
Because let's say you're Tom Hanks. And you get TomHanks@Lemmy.World
Well, what's stopping someone else from adopting TomHanks@Lemm.ee?
And some platforms minimize the text size of platform, or hide it entirely. So you just might see TomHanks, and think it's him. But it's actually a 7 year old Chinese boy with a broken leg in Arizona.
Because anyone can grab the same name, on a different platform.
This solves the issues of having the same username across all platforms, assuming you host an instance for every platform you want to use. And also mske those domains private. But it doesn't address that same username being used on another instance/domain.
Like imagine someone had the usernsme Ada@someoffensivedomain.social and was impersonating you. If you made your living off your name, an imposter would affect your image.