A central account instance rather defeats the point of a federated system.
With federation it’s ensured that any single instance is only a small part of the whole, and that if any instance goes down (or worse, goes rogue and becomes a bad actor) then the impact of that is minimised. All users being registered on a single instance is akin to putting all your eggs in one basket.
I do totally understand from the perspective of new users that it’s hard to understand what to do or how to do it but that is a problem that could be better addressed with clearer onboarding. e.g “Choose any one of these recommended instances to sign up. It doesn’t matter which - you’ll be able to see the same content and communities across all of Lemmy no matter which you pick”*