In other words, I can only protect email from my provider as long as everyone else on the email protects it from theirs. And it only takes one gmail user on the thread to ruin it for everybody.
If I'm corresponding with people who also care about this kind of thing, we aren't using email. If we absolutely have to, then we can encrypt it with PGP.
My email use cases are: (1) b2b professional emails, which you expect to be retained for compliance, etc. and (2) transactional emails, which my provider unavoidably has access to and which I protect with strong passwords and 2FA. (That said, I would like to obscure the list of my internet accounts from google, so maybe one day I'll migrate my transactional emails somewhere else.)