When they are drafted, they have 0 pass attempts.
A completely healthy QB who was highly recruited to be a top 5 pick who was sitting behind a probowl QB would still be that same QB 2 years later. What has changed other than he is now 24?
Do you think if the Colts drafted Luck and Manning stayed with the team, he wouldn't be worth a 1st round pick 2 years later?
For one thing his contract. If rookie that sits for 2 years and then is traded to a new team. His new team only has 2 years to evaluate him before they have to offer a big contract. If you figure it takes 2 years on the field before a QB to get comfortable with the NFL speed, playbooks, different looks, and NFL defenses. then those 2 years really are a lot more valuable then you seem to realize. Even though he was in a system for 2 years once he goes to a new team he has to start all over with new coaches, philosophes, playbooks, system, and training plans. I really think sitting on the bench is not nearly as valuable as a lot of people think it is.
Also your Luck / Manning example is crazy! A more realistic example would of been ... Do you think if the Colts drafted Luck and
Painter stayed with the team, he wouldn't be worth a 1st round pick 2 years later?
You are suggesting the trade value of Trubisky if he can not win a job from a backup QB and not the trade value of Trubisky if he can not win the job from a HOF QB who played his entire career with the team that drafted him.