Lets first of all, assume any major college Head Coach and any NFL coach are high in football acumen. They all know the game. So it really comes down to so many other factors outside the X's and O's.
So consider this . . .
The process or system of being a Head Coach in the NFL is totally different than in college. In college, you build a program.And, really, the only hinderance is the amount of money and priority a particular school wants to place on it—at Alabama, neither of these are an obstacle.
In the NFL, you have to build your team within very prescribed boundaries.
Saban is great at the program building, especially in an environment where he has everything one needs to be successful. They give him everything he asks for and more (Say he was at Syracuse, do you think he would be as successful as he is now?). Alabama is the perfect fit for a person like Saban. And to his credit, he knows that. He knows there are very very few other places where he could have success like he is having there.
And, most relevant, he knows the NFL is not that kind of place. It is a totally different type of role. There are way more limitations for an NFL Head Coach than there ever will be at a football factory like Alabama.
Could he be successful in the NFL? Sure, he knows the game as well as anyone. But is the likelihood as high in the the NFL? No. He's smart enough to know that, and he is in a position where he gets to decide.
He is right where he should be.