How would encrypted emails work in a client like Thunderbird? Would it normally be handled correctly if POP/IMAP was available?
But... those aren’t particularly friendly and thus aren’t widely used. The main usability issue is a fundamental one, namely that management of crypto keys is hard.
Hearing that Protonmail requires their own client suggests to me that they’ve given up on the standards due to usability issues, and have instead adopted a managed key model like Apple’s iMessage.
iMessage is end-to-end encrypted, but Apple manages keys on your behalf. It’s not a bad compromise between privacy and usability depending on your threat model.
But, at this point I’m deeply suspicious of anything that isn’t standards based or that locks me into a particular vendor’s software. I’m therefore skeptical that we’ll satisfactorily solve email privacy for a majority of people in my lifetime.
EDIT: Thunderbird would work with Fastmail for encryption using PGP or SMIME (at least I think Thunderbird supports SMIME). Protonmail wouldn’t work, I’d guess.
Thunderbird supports SMIME natively, just like most of email clients. But of course there are some usability issues, like you need to enable encryption each time per email, or require it for all emails, there is no middle ground like "encrypt if I have keys, do not encrypt otherwise".