The reality of the situation is that you can't remove the human factor from security. So someone copying your email to someone else is a human problem that can't be fixed - someone could just as easily photograph the screen.

The reality is email will continue to be used, and there is a use case for being able to send an email securely to another person.

EFail was pretty bad, but only affected HTML email.

Having a modicum of backwards compatibility is how to encourage transitioning to new tech, so the RSA implementation makes sense.

I must say this is the first decent alternative I've seen for GPG instead of rants about Signal and specialised tooling that just ignore the issue that folks want to be able to send secure emails to each other.