>* An enormous installed base of clients that won't do encryption, meaning that at best you're attempting to tunnel encrypted messaging over an unencrypted transport.
All encrypted communication on the internet is 'attempting to tunnel encrypted messaging over an unencrypted transport.' That's literally the entire point. I don't care that my messages are broken down into unencrypted IP packets, because my messages are secured by PGP.
>* A protocol that leaks metadata, including some message content, at the envelope layer.
Could you elaborate more on this? I don't really understand what you mean.
>* Hundreds of millions of users that primarily access messages through browser clients that can't meaningfully implement crypto.
They don't. They could, but they don't. There's no lack of capability.
>* An archive-always UX that ensures that huge amounts of plaintext are scattered around the Internet by both senders and receivers.
That's nonsensical. Nothing about archiving data requires that it be stored in plaintext. Again, that's basically the whole point of encryption.
>* An unencrypted installed base that ensures encryption will be opt-in for the foreseeable future, meaning that users will routinely reveal plaintext accidentally by, for instance, quoting messages and forgetting to encrypt.
That's just bad UI design. At the end of the day, your intended recipient has to be able to actually read the plaintext, and they will always be able to reveal its contents, if they want to. No matter what you do, they can always just take a screenshot or show someone their device's physical screen.
Making it easy or difficult for them to accidentally reveal it is entirely about UI design. If you quote encrypted mail when composing an unencrypted mail, make the screen go red!
>* End user demands for things like search that can only be delivered efficiently at scale by databases of plaintext (most likely at centralized servers).
Rubbish. GMail is clearly what you're mainly referring to, and the idea that GMail requires big plaintext databases for spam prevention or searching is bullshit. It doesn't.
>All these problems are probably surmountable (with enormous, concerted effort).
Most of them aren't real problems, and those that are, are quite easily fixable.
>But: why bother? Email is just one of dozens of messaging systems available to Internet users. Better to move sensitive conversations to things like Signal, WhatsApp, or Wire --- the double ratchet construction is designed specifically to make IM-like protocols secure even when conversations are sporadic and last months.
Sorry, but I have no interest in proprietary messaging protocols. I want to use email. I like email. Everyone understands email. Git understands email.