There are different kind of developmental guys. You don't take a guy who is at least 2-3 years away from sniffing NFL ability just because you have a starter. If you're going to pull the trigger, you take the most polished guy you can get and he's better off for being able to watch and learn.
Prescott is a football player with good athletic ability. He is never going to be an NFL passer. He may one day be a back-up, but that's about where his potential stops. I guess what I am saying is, there are far better and more feasible developmental guys in this draft, even in the middle rounds, than Prescott.