I don't really see the link between the email you posted and efail, other than the fact that PGP encrypts the whole message. I don't understand how, if it encrypted smaller parts of the message, efail could have been avoided.
No competent engineer would accept in 2019 (or, for that matter, 2009) a new cryptosystem that functioned the way PGP does.
Of course, some things ought to just be replaced (S2K).