SFTP shares only those three letters with FTP but is otherwise a completely different protocol.
It's also a counterpoint to one of the OP's points which is that trying to secure email is a dead-end that we should give up on. It may be possible, though very difficult, to transition to secure email. But it would have to be a brand new protocol not tacked-on to the current system. The way that SFTP is the appropriate upgrade path for FTP and not FTPS.
The interface of sftp is largely the same as ftp. You can give someone an sftp client, username, and password and they wouldn't be able to tell the difference.
A new e2e protocol that works just like email does would be excellent, but we don't yet have that. Signal &co are chat apps with a different feel and purpose and other than matrix and xmpp not federated, which is a non starter for many entities.
There are some things we could easily give up to get encrypted mail. For example, setting a max delivery time to allow ephemeral keys.
But there are other problems with encrypted messaging, and I'm particularly thinking of spam. How does Signal/WhatsApp manage to keep a lid on it? Would that be possible in a federated system?
