Occasionally, a handful of exceptional players can raise a team to new heights. Perhaps they are particularly diligent, develop simultaneously, or there is some other idiosyncratic event (Walker spread offense, Moneyball).
But in the medium term, it's about coaching. There is a reason that they make so much money. Alabama was mediocre (albeit great history), prior to Saban. Likewise Oklahoma before Stoops, Stanford before (gritting my teeth) Hard-blow.
Coaches recruited the players, condition them, train them, mentor them, teach them, direct them in practice and in games. This means, that teams I dislike like Mich and Clemson, with loathsome coaches, will continue to succeed due to coaching quality. ND will continue to wallow until they get a home-run hire.
Lifetime mediocre to above average coaches like Ferentz will have occasional good years, then revert to the mean. They never get fired and no one will hire them away.
We've seen the best and worst of Fitz. Hoping for multiple B1G championships is a (crack) pipe dream. His range is 7-5, plus/minus three. That's it. He is a lot younger than me, so this is the NU football that I will get for the rest of my life. I've come to accept it