The point is that instead of requiring end users to verify public keys, it's better UX to give them the ability if they want, but not require. I remember an email standard that sent public keys inside the emails, and the replies are encrypted with that public key.
Sure, MITM is possible, but it's easy to detect, at the same time the UX is easy to scale to billions of people.
That's autocrypt. It is awesome and should be default on all email clients. As part of the autocrypt standard MITM is not detectable as that would just provide users with information they could do nothing about. Autocrypt is fine just as long as the users are made to understand that the privacy protection is a lot less than the protection they would get from a PGP key they actually got from someone directly. No UX in the world can overcome that incontrovertible fact.
Sure, MITM is possible, but it's easy to detect, at the same time the UX is easy to scale to billions of people.