Doug Flutie actually reinforces my point. That guy was a great college QB who spent his career in the NFL mostly as a backup.
Actually he may be a good comparison. He couldn't get a starting job in the NFL because he was said to be too short. He became a Hall of Famer in Canada and then was a starter in the NFL at an age when most QBs are retired. I'll always wonder what he could have done in the NFL in those intervening years. I think he played well enough during his second NFL run to at least dispel the height concerns.