Does Ian Book have a chance at the job?

What makes a NFL caliber QB, or any other position for that matter. I have seen QB's with tons of athletic ability, way more than most starting QB's in the NFL right now, that have never even made the practice squad, most never even given a chance to compete. There have been big names from major colleges that never made it in the NFL, while no name QB's from lower tier colleges have been successful. Mike Buck, University of Maine, could throw the ball 90 yards from the kneeling position, but never made it pass a backup. Heard he was not very smart and could not learn the offense. So why have him as a backup?

There have been very cerebral QB's, but not athletic that were never given chances. So what does it take to be given a chance to succeed in the NFL? Just curious.