I'll echo what others have said: Saban retiring or a finding of NCAA violations resulting in extreme sanctions would certainly do it. I'd like to pose a couple of other possibilities; first if Saban were to stay too long we could expect the machine to come apart (look at Bowden's last decade at FSU or Paterno's final decade at PSU). Bama's success under Saban, much like our success under Osborne is largely a result of a head coach enforcing a culture of excellence with his staff and players, if Saban starts losing the energy to do this Bama will begin to slide little by little until the wheels fall off completely. A final possibility is that Bama remains an excellent program, but someone surpasses them. If I had to bet I would say that Saban retiring is the most likely cause of the end of Bama's present golden age.
As to Dabo taking the job, I think it's possible, but there are a couple of things that make me think it's not likely. First, he has already established himself in a great position, why would he try to follow up what Saban has done? On the flip side, especially if Clemson wins on Monday, he might feel compelled to see what he can build and maintain in Death Valley.