many folks don't realize the problem with this is since his first year, he really has not played well. That first year he kind of caught some people by surprise and his mobility was such a factor, but since that time, he can't lead teams to wins and his ability to make the throws necessary has been exposed.
Now, given that, there has been plenty of backup QBs that stayed in the league after performing poorly just because they had experience and coaches were too scared to play an unknown.