Of course it’s more difficult as a younger player. But you said every player not named Mahomes struggled.
Dak went 13-3, 3600 yards, 23 TD and 4 INT.
Lamar lost one game as a starter and put up similar production as his second year.
These guys all get better and it gets easier. But the fact is, the guys who stick it out in the modern NFL are good from the beginning. The guys I named are examples of that. Your rebuttals with guys who were rookies 40 years ago doesn’t exactly show otherwise. The NFL is very different today and QBs come into the league way more polished.