Fair point, but I was thinking something like Signal, WhatsApp, or the like. So, I like to think of email as a bit dated, not only in security but in model.
That said, the beauty of email is that you have a unique identifier that doesn't force the other party into your walled garden.
If we could develop something akin to Signal, but that didn't require the second party to have to sign up explicitly for that service, I think that could finally start the process. Which, of course, would also require some level of initial adoption, probably the hard part.