I say the same about the other QB's you mentioned and I include Kelly's last couple years in that list. JP Losman looked at one time like a QB capable of leading fourth quarter comebacks, having done it numerous times. Then he took way too many hits and was done. I feel that Rob Johnson came here as a very talented but flawed quarterback, but horribly regressed.
Add to that, the list of offensive coordinators that failed here, but went on to great success elsewhere- Bruce Arians, Kevin Gilbride and Dan Henning all come to mind.