I don’t think that’d work, with Lemmy being a federated model, not a fully decentralized one.
How do you handle the actual login? Does that mean every server has access to your password hash? Or do you overhaul the account system to use something like a private and public key, with the user needing to store and transfer the private key to every device they use?
And what happens if two people register with the same username on two instances that aren’t federating? Do they somehow need to still communicate with all other instances in the network they operate in, to prevent that from happening? Because the alternative I see is the login being random in some way or tied to the instance, in which case you still lose the impression of a single service.
If I’m not mistaken, right now anybody could host a non-federating Lemmy instance, if they just wanted a small private community in this style. To my understanding, that’s the idea behind federation, and a founding concept of Lemmy - it’s not a giant service distributed across trusted servers, but a network of smaller communities that communicate with limited trust.