I think it's safe to say that if Cam goes it will be because he isn't winning. The 49ers went with Alex Smith for 7 seasons and like you said he wasn't benched for performance reasons. Chicago knew that they were getting a starter in Cutler and Denver got a really bad deal out of that trade but Cutler was still known around the league as starting QB material. If Cam gets to the point where we are willing to throw him away as the number 1 overall pick I think there will be serious questions as to whether he is actually capable of successfully leading an NFL franchise. The only comparison I can think of where a career was resurrected is Brees. Maybe you could also throw Kurt Warner in there because he bounced around from team to team after he left the Rams until he was finally successful in Arizona. However, one thing I'm not worried about is the Panthers letting Cam go and then him turning out to be a start somewhere else. I think he's getting too much of a pass from this board and there are serious questions in my mind about his ability to be a starting QB in the NFL.
Brees wasn't let go because of performance reasons. He was successful in San Diego. They drafted Phillip Rivers in 2004 because Brees was struggling. Then all of a sudden he exploded in 2004 and the Chargers went 12-4. He had an average year in 2005 and the Chargers finished 9-7. He tore up his shoulder in the final game of 2005, and the Chargers decided to move on to Phillip Rivers. The shoulder thing was also the main reason Miami passed on him over Daunte Culpepper. So I'd say injury paired with the fact they drafted his replacement already was the reason he was let go...not performance.
If Cam is let go, it'll either be because of a) injury, b ) his replacement is drafted or the worst case scenario c) he doesn't play nice with the new head coach.