Many emails aren't sent unencrypted any more — just not E2E encrypted. It's harder to stop an active MITM from downgrading the connection, but the bulk of non-spam messages to my server come in with TLS. And while it's not going to be possible for most people, I have pinned most of my larger destinations to require TLS with a suitable certificate, so I can have confidence that my outbound email won't transit the Internet unencrypted.

Obviously if you're a client of a big hosting service that you don't trust then E2E has value. But that's not the whole problem, or the whole solution.