The issue with email is that too little is specified, and is instead just left up to providers and clients.

This specification vacuum forces centralization because the only way to build essential usability features is to own both the mail server and the mail client.

If email had evolved to move with the times such that basic QoL features were part of the spec rather than proprietary extensions, then it could have stayed decenteralized.

Contrast with what happened on the web. Yes it's imperfect but there is a standard that evolves to move with the times and there are multiple implementations of that standard.