I feel too many QBs entering the league are not ready to play right away, and risk having their confidence and potential destroyed as a result .
You're right that it would not have mattered for a lot of them, but there also probably were many who would have had far better careers had they been given a year or two to get acclimated to the league before getting thrown to the wolves.
One can argue exactly how much Brady and Mahomes were helped by practicing, sitting and watching for a year, but they definitely were not hurt by it.
Granted, they also had the added benefit of having successful veteran QBs ahead of them to learn from, which is something many rookie QBs don't have.
Even if those veterans were not the ideal mentoring types, the younger guys could learn a lot by observing how those veterans prepared and handled themselves at the QB position.