By that logic no rookie should ever start.
Stroud has everything to prove still, no question about that. He's played zero snaps of a real NFL game. The criticism of him is one thing, and most likely warranted since he's never played an NFL snap, but to think he should sit and watch Mills for a game or 2, 3, 6 or whatever, well, that just doesn't do anybody any good.
Sure, Aaron Rodgers sat for 3 years but in his 1st 11 starts he had;
63.5% completions, 17 TD, 9 INT, 90.5 passer rating.
Davis Mills 1st 11 starts;
66.8% completions, 16 TD, 10 INT, 88.8 passer rating.
Point being, there is no tried and true formula when it comes to sitting vs. starting right away. Rookies, or 1st time starters, are going to struggle and make mistakes. Period. Talent and how they grasp the game will determine the rest.