Before even the flawed STARTTLS was a thing, people wanting to have private communication over unencrypted email would simply exchange public keys and use PGP to create encrypted messages.
If E2E were not available for any reason whatsover, we could go back to that as necessary, though might lose some extra benefits of integrated E2E platforms like perfect forward secrecy (or maybe not; I'm not really up to date on my crypto).
With a standard and open protocol enforced, we could create a FOSS client that automatically pre-shares a public key and de/encrypts messages before sending them, so the UX would be much better than the awkwardness of early PGP.
If E2E were not available for any reason whatsover, we could go back to that as necessary, though might lose some extra benefits of integrated E2E platforms like perfect forward secrecy (or maybe not; I'm not really up to date on my crypto).
With a standard and open protocol enforced, we could create a FOSS client that automatically pre-shares a public key and de/encrypts messages before sending them, so the UX would be much better than the awkwardness of early PGP.