Saban is a better coach now than he ever was at MSU or LSU. At those places, he was always looking for the next challenge. Now, he's figured out what he loves to do and is doing it to the best of his ability. He seems more interested in building a legacy(that's his challenge) than changing gears. Of course, Ms. Terry might have some say in that too.
I think that this is what Nick Saban has always been about. He sees life as a process and is now reaping the benefits of allowing himself to go through it. He has been at Bama longer than any other stop along his coaching career. That is not to say that he won't leave for more money (God forbid), but he genuinely seems happy (well as happy as Nick Saban can be...lol) in Tuscaloosa, with these players. I think he stays as long as he continues to enjoy it. I don't see him wanting to go somewhere else to start over again. He's been there and done that 3 times now (Toledo, MSU, LSU).
Maybe I am drinking the kool-aid, but I just don't see him going anywhere until he retires.