Right, but email is really not all that great, neither on a practical nor technical level. 99.99% of all users doesn't run their own mail server so in practice it is not decentralized. I would guesstimate that 50% or more of all mail traffic is handled by Google, MS, Apple, and a few outer mega corps. And for most people Facebook, Slack, and WhatsApp are way more convenient communication methods than email.
Your criticisms have some validity, but email is in a sense objectively great in that it's the thing that "won."
I believe there's a universe of difference between ONE provider and a few megacorps plus weirdos like me who pay for my own domain; I actually think that would ALSO be a good model for twitter-esque microblogging the way it pretty much works for email.