"You keep plaintext archives and drafts of your messages, because that's a fundamental feature of email clients going back 3 decades."
No it's not, you don't have to keep it at all or in plaintext and it's irrelevant for new clients supporting encryption, their UX have to be redesigned anyway. So it's a UX issue at most.
"Your peer accidentally forgot to encrypt a response and quoted your own plaintext back to you."
Again, just a UX issue. Although all messaging apps actually have incentives to provide a UX that lets them spy on most people's communications or be open to add that possibility some time in the future.
"that has your security one surreptitious Javascript injection on an XHR call away from complete collapse"
This is a problem of centralization that you are trying to ignore and none of those messaging apps can solve it. Any centralized system is one tiny change away from a complete collapse. It can also be shut down by the state just to force people to use plaintext or backdoored alternatives. The problem is even bigger than it looks, even if you make a completely decentralized protocol there will still be incentives to centralize as much of it as possible to make money and still leave a strategic possibility to spy on everyone also for money. Makes sense?
No it's not, you don't have to keep it at all or in plaintext and it's irrelevant for new clients supporting encryption, their UX have to be redesigned anyway. So it's a UX issue at most.
"Your peer accidentally forgot to encrypt a response and quoted your own plaintext back to you."
Again, just a UX issue. Although all messaging apps actually have incentives to provide a UX that lets them spy on most people's communications or be open to add that possibility some time in the future.
"that has your security one surreptitious Javascript injection on an XHR call away from complete collapse"
This is a problem of centralization that you are trying to ignore and none of those messaging apps can solve it. Any centralized system is one tiny change away from a complete collapse. It can also be shut down by the state just to force people to use plaintext or backdoored alternatives. The problem is even bigger than it looks, even if you make a completely decentralized protocol there will still be incentives to centralize as much of it as possible to make money and still leave a strategic possibility to spy on everyone also for money. Makes sense?