I haven't seen anyone proposing actually doing it, but I think a lot of people are rightly pointing out that bad actors, black hats and the Russian mob are going to have a field day with their ability to do so.
I’m not sure how you can conclude the speculation is “right” without engaging with the fact that this hypothetical is addressed directly in the threat model document and hasn’t been pulled off successfully against any of the other services which do similar scanning. Why can’t I buy compromat as a service for your Gmail account?