It’s obviously not perfect. And yes, different QBs have different talent levels. No duh.
But what you’re asking for is impossible to find. It’s impossible to see what happens when a QB both starts as a rookie and sits as a rookie. Those things are mutually exclusive. The best we can do is look and see what happens on average when one starts and when one sits. And based on career outcomes, it’s better to play early. That’s why NFL teams have their rookie QBs play early.
You also argue that my analysis is flawed because we don’t know how much better Manning and Co. would be if they sat a year. But in the same post, you use guys who didn’t play as examples of why it’s better to sit. But you don’t acknowledge that it might be better for them to have played early.
The fact is, you think you’re right based on 20 year old example. But when you actually look at statistics, they show you’re probably wrong. They obviously aren’t perfect, but it’s better than, “well, Joe Montana sat 40 years ago so I’m right.”