While Cam is indeed our franchise, he's not yet a Top 5 QB. So the better question to ask is, has a starter ever willingly gone on to a different team. Many come to mind: Brees, Orton, The Golden Calf of Bristol (from Playoffs to laughing stock), P. Manning, Montana, Warner, Delhomme, Pennington, Bledsoe, McNair, Favre, Palmer, Campbell... you know what, it's almost 4 AM and I'm tired, I don't even know what I'm typing or if I'm making sense. But the fact is, great QBs have indeed changed teams. The elite usually don't though. So if Cam continues to trend toward being elite, he won't go anywhere. If he just stays great on an average team, anything is possible.
It doesn't, get some sleep.
Literally everyone you listed was either effectively forced out or was traded. All left against their will.