I don't understand this argument.

I understand that PGP might not be the way to go. I understand that metadata might leak. I understand that every cypher will be broken, eventually.

But what's wrong with trying to send a an e-mail with contents you want no other party than the recipient to read, again, provided the recipient doesn't share it with other people.

For the record, I don't send encrypted mail. UX for these is generally terrible, hard to verify sigs, etc. I am genuinely asking what part am I missing. Is there a fundamental reason that encrypted e-mail cannot work with current e-mail protocols? (but replacing PGP if needed)