What happens to Luck if he sits behind Manning?

The popular opinion is that keeping Manning and Luck would be the worst thing that this franchise could do. One non-salary cap/non-talent acquisition argument is that Manning won’t give Luck meaningful snaps, so how could he get better? I don’t understand this argument.

How did Aaron Rodgers get better sitting behind Favre? Rodgers sat on the bench for three years. He comes out and is immediately elite, and after a year, playing at a hall of fame level. Rodgers got to the stratospheric level of Mannning, Brady and Brees and did it in his fifth season after just one full season starting. It took Manning 6 years, and Brady and Brees both 8 years to get to that level, with the latter two sitting their rookie year.

To reiterate, Peyton Manning started from day one, and it wasn’t until his sixth season that he was playing like a hall of famer. It took him longer than Rodgers even though he took EVERY MEANINGFUL SNAP!

Is there something that I’m missing here? Something that I’m overlooking? Why do people think that Andrew Luck will stagnate on the bench? If anything, he’ll have an opportunity to adjust to life in the NFL, become familiar with the offense, and gain the respect of his teammates without having to also shoulder the burden of starting for an NFL franchise.