I don't see why you can't understand. When a QB isn't challenged and has a free ride his play will tend to drop. When faced with the real possibility of being replaced they step up their play. It has happened 4 times recently. And it certainly did have an effect.
You laugh at Osweiler, But Peyton was coming off neck surgery and Broncos didn't even know if he could still play. The drafting of Osweiler gave Peyton a short runway to get his game back to top level. He responded.
Brees was crap. They drafted Rivers and all of sudden he became good with the fear that he was going to loose his job to Rivers.
Smith average, but pushed to the top of the league when facing loosing his gig.
Brady great but not able to dominate for 10 years, draft a replacement and he wins 2 more Super Bowls.
Laugh all you want. If you think you can't be replaced you will perform at a lesser level IMO. And there are 4 cases that support my theory.