Oh, I don't know about that. Sure seems like production mixers are getting thrown under the bus on set with great frequency these days. Camera rehearsals are for the sound dept too. Instead, the poor boom op or utility is still wiring the actors up and the AD is calling the roll. If you can't tell, I used to have a sound mixer for a roomate.
You could say the same thing about cinematography and art direction as well.
"We don't have the time to fix that, lets fix it in post"
It's a lot harder to fix audio in post then picture. Audio doesn't have the dimension of picture, you can't separate frequencies and make it recognizable to the human ear. So it's extremely limited compared with picture. ARD or Automated Dialog Replacement, use to be a very main stream thing to do. It was almost required before we had the wireless technology we have today, where mic's can literally be placed on talent. Where the sets are quieter then they've ever been, due to silent cameras and a lot of careful modern techniques to reduce foot steps, camera dolly noise, etc. Sure, there are extreme cases where dialog is just unusable and ADR is required, but those happen less and less today. The set performance is always better anyway, it's rare an actor has the same passion about his role in the cold, dark, VO booth then they did on set, in costume with cameras rolling.