I like Herbert as a QB, but in doing projections myself, he just doesn't end up much higher than 16-18 for me. Harbaugh has always led a slow paced offense, even in SF, and the volume is just not there to support anything more for Herbert.
FYI, giving Herbert 3,800 yards and 42 TDs as posted above makes him QB5 in my rankings. But those projections are not realistic at all IMO. People point to his 2021 season as a comparison, but there is no way a Harbaugh offense has that kind of volume. Herbert dropped back 674 times that year. Last year? 504.