Given what we know/have seen of Coach Saban's approach to life, leadership, and football, I can't help but wonder if he might approach his exit transition differently than most retiring coaches/outgoing leaders might. I can imagine him seeing his legacy as including what happens after he leaves, at least in the short term.
I think it would be foolish to expect a continuation of the level of winning Saban has attained, but I don't know that a precipitous drop is inevitable. I'm thinking of the transition from Steve Jobs to Tim Cook at Apple. It was hard to imagine Tim Cook being able to follow Jobs' level of success, but he has.
In the past I would have said that any fanbase would be impatient with not winning at the level the outgoing coach won at, but I think Saban has actually made that a little easier for his successor. Saban's level of winning is so unusual that no one expects that to continue. The expectation will be more about the product on the field, not the outcome of the product. As long as the product is disciplined, has character, and fights hard, winning will take care of itself AND fans will be patient. However, if not winning is accompanied by undisciplined teams with character and effort issues, then that team's coach isn't going to last long.
Long way of saying that if anyone is going to set his successor up for success, it will be Saban. It would very uncharacteristic of him not to.
(This assumes the administration lets Saban have input and influence on the decision...which surely, surely, surely they will.)