The physical world equivalent for a fully decentralized fediverse then is that every user sends mail to every other user’s house, as needed, similar to how sending letters works in the physical world. This is decidedly not the case with a fully decentralized ATProto. The physical world equivalent would be that every user had their own house at which they stored a copy of every piece of mail delivered to every other user at their house.
If this sounds infeasible to do in our metaphorical domestic environment, that’s because it is. A world of full self-hosting is not possible with Bluesky. In fact, it is worse than the storage requirements, because the message delivery requirements become quadratic at the scale of full decentralization: to send a message to one user is to send a message to all. Rather than writing one letter, a copy of that letter must be made and delivered to every person on earth.
Christine Lemmer-Webber
How decentralized is Bluesky really? (Dusty Cloud)
Scott Boehmer 