Nobody outside tech cares about decentralization or federation.
At some point, everything converges to centralization.
No amount of Mastodon servers or any fediverse self hosted instances spun up will change that.
There is a reason that mastodon.social is the biggest instance and that they couldn't close registrations to promote decentralization.
Hell, I would even say that threads is the biggest mastodon instance.
E-mail is really ingenious. It doesn't even define how to send and receive messages in the same standard. One standard is just for delivering messages; you have to figure out how you're going to receive them separately.
Of course most people have an e-mail address hosted by one of a handful of large companies. But you don't have to. And if you buy your own domain, changing providers is easy. Delete your old mail on the old server, upload it to the new server, people can still contact you the same way they did before.
I'm not on social media, so I don't have any dog in this fight. But all the properties of a good decentralized/federated platform are already there in decades-old protocols.