Email is decentralized. It's mature enough that there's a bunch of trusted providers out there and you can use it without caring about the technical details, it's basically a perfect example of a successful mature decentralized service.
Seriously: Transforming spam filtering into a black box service has decommoditized email and everyone knows it.
email isn't a social network and lots people outside of HN don't self host them as such.
Business email is very dependent on the fact that it’s an open protocol.
And so the word comes down, we have to move to Gmail. It’s believable decision making, but that’s how it works.
People don't self host email because 1) the protocol doesn't make it easy to deal with incoming spam on a small scale, and 2) giants like gmail tend to automatically flag unique email domains as suspicious. Which is a valid criticism of the federated model, or rather the people using it.
If more people found value in decentralization, in theory they would also be more critical of their node management, opting to reject anti-competitive behavior in a democratic way. Unfortunately, for democracy to work, citizens have to be educated and fight for it. If they don't, it's easy to have centralized, ostensibly totalitarian walled gardens.
We just need to make sure fediverse accounts are easily transferable between nodes in a standardized way, something I don't think mastodon does well, and email doesn't do at all.