Official instance admin and community mod roles are denoted with badges, though. That's consistent across every instance already.

I could change my name to admin and post here, but it wouldn't give me any superpowers. Looking at my remote @admin user from a federated instance wouldn't badge me as an admin for that instance.

I don't see the problem or see how that affects anyone. There can be an @john on every instance. Is that confusing for users, too? Should every instance coordinate and make sure there's only one @john to rule them all? Who is the best John, and how is that determined?

That's just how the fediverse works.