This is exactly what I was going to say. Nice work none the less. There is a very strong correlation for sure, but there is wiggle room for confounds. What exactly is the driving force behind these "successful" qbs and "unsuccessful" qbs? Is it their own ability, or is it their system? There are very few that can stand on their own and say that it's their TALENT and not the system. So can this really be about how good the QB is or could it possibly be because of outside influences, in this case, the system. Maybe its the QB coach they have, maybe a combination of all. What if EJ fizzles out here, is it becuase he doesn't have the talent, or was he not put into the right situation?
This is a strong correlation, but I wouldn't stand behind it wholeheartedly, there's too many questions and possible confounds. It would be very tough to control for these conditions. It would likely have to be a case study, ie, Peyton Manning. He left the Colts, is in a very different system in Denver, and still wins. There's a very strong argument that he stands on his talent.
Anyway, very well thought out study. Good work!