The telltale was the churlish behavior in press conferences. If you can't answer a few tough questions without snapping, how are you going to deal with the pressures that position demands? Well, we saw he couldn't handle the pressure. The pick at the end of the Super Bowl was a blip, then there were the late game collapses against the Seahawks and it has been a spiral since.
The only things that kept him afloat so long were idiotic defenses like the Packers playing man. As soon as the defense turned and ran with receivers, Kap took off. Or he could wait for a receiver to break open late. He never improved any facet of his game. Just a waste.