It's not exactly PGP's security model, but:
- if you use SMTP-via-TLS and (IMAP/POP/MAPI)-via-TLS, attackers really can't intercept your mail
- unauthenticated encryption still protects your mail spool at rest.
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.
Not necessarily. Mail can be intercepted when transferred between MTAs which seem to be often vulnerable to STARTTLS stripping due to a fallback mechanism. See "Neither Snow Nor Rain Nor MITM... An Empirical Analysis of Email Delivery Security"[1].
