However, I disagree that most QBs are given up on too early. Coaching staffs are watching these guys every day in practice. If you've got "it" then "it" will show itself. Mark Sanchez's rookie year was terrible by statistical standard, but it doesn't really matter because he showed enough flashes of potential to confirm that he has the ability to be a good quarterback in this league. Brady Quinn hasn't shown that he does anything well.
While there will always be a story of a guy like Jake Delhomme or Kurt Warner slipping through the cracks when all they needed was a little more time to develop, the truth of the matter is that most guys just simply won't ever get it.