Email isn't decentralized, and that's fine

Email != SMTP

A better title would be “Sending email isn’t decentralized”

A mailbox is POP or IMAP and there are many decentralized ways to run these.

SMTP is delivery, sure there are many gatekeepers, because spam is so bad. It comes down to trust. You might have to pay for transit if you can’t get the world to trust your SMTP server, which is hard for many good reasons.

"Anyone can run an SMTP server and send email. But actually getting messages delivered depends heavily on a handful of powerful gatekeepers." Reminds me of the PhD system. Anyone can write a dissertation and get a doctorate. But actually getting an agreed on date when you are "ready" and can graduate depends heavily on a handful of powerful gatekeepers.