I think decentralized social media isn’t really analogous to your email example and is going to be much more confusing and frustrating to an average user. I may sign up with Gmail, but I can still in almost any use case send or recieve an email to whoever I want at other domains. The fact that someone is @X or @Y doesn’t matter to most users–it’s just an extension of the address.

Decentralized Social Media I think would be viewed more like having @Gmail001 through @Gmail100, with users frustrated wondering why they can’t receive an email from their cousin who is on @Gmail088. We’re both on Gmail, what gives