Encrypted mail that only works when attackers can't intercept your mail might just as well not be encrypted.
Of course, attackers gaining access to your mailbox is still game over, due to password resets etc. via e-mail; but "attackers can't intercept your mail" is a more realistic assumption in 2018 than it was when PGP was designed.
[1]: https://conferences.sigcomm.org/imc/2015/papers/p27.pdf