Injury?
As for it can't be proven, we can make educated projections by the body of work from the journeymen QB in the league as to the likelihood of success in any system. Tape doesn't lie. Can they read defenses? Can they move in the pocket? Are the accurate? Do they make quick decisions? Can they make the throws? These are just some of the analyses we can do on existing talent that is system independent. We've all seen enough of Bryan Hoyer (as an example) to know he ain't passing for 5000 yds in any system, with any talent around him. Especially with a very limited run game to support him.
None of this answers my questions.