the entire point of encryption is that you don't trust the channel you communicate through, that's what it was invented for, communication across adversarial channels. Distrust is the only condition under which you need encryption.
In addition from a practical POV it's if anything the reverse is the case. Email encryption is larp security because plain text is the default, leaks metadata and its interfaces make it trivial for people to leak entire conversations. If there's one technology where you should just assume your messages are public, it's email before someone copy pastes or wrongly forwards your encrypted communication to fifty other people.
Private message encryption makes sense because it's now a default, information exchanged is usually personal, and the problem isn't just Meta but law enforcement extorting your data out of their hands, which encryption in the real world has prevented a few times now already.
