This is not the point of the commenter you're going back and forth with on the thread, but as I read this it occurs to me that some version of the plaintext rep

This is not the point of the commenter you're going back and forth with on the thread, but as I read this it occurs to me that some version of the plaintext reply problem exists in real life regardless of protocol. That is, once the recipient has plaintext, we don't know what they're going to do with it. They could read it aloud in a crowded place. They could blab about it to all their friends. They could pass it verbatim to somebody else. Or more innocuously, they could store it in an insecure database as in this commenter's scenario. Now, if you're using something better than email, the protocol itself won't be the problem and it's less likely (or impossible through the protocol itself) to be an unintentional plaintext transmission, so this fact is pretty moot and not at all a justification of how email clients might behave. Nobody building these systems has to necessarily be concerned about protecting against "social" types of plaintext leaks.