I feel like Bama's fanbase has essentially reached the point where winning a NC is expected and anything less is a huge disappointment. That fanbase is going to absolutely meltdown when they hire the next guy and he posts back to back 9-3 seasons or something equally "good but not great."
Saban is easily the best CFB coach of all time and the next guy, no matter who it is, won't even come close to matching his success. One of my high school friends is a Bama fan and actually has a realistic perspective on it and said that he knows it's not going to last forever and is just trying to appreciate the run while it lasts, but I think a huge percentage of their fans will lose their minds once they start trending downward, whether it happens under an aging Saban's watch or happens under the next guy.