I think there are only two things that could cause Coach Saban to call it a career:
1. Health issues/age. Although he's not that much younger than Coach Bryant was when he died, he's clearly taking better care of himself. When he says that he wants to go on as long as he can do the job, I take him at his word.
2. Very unlikely, but if different UA administration wants to interfere in the football program, I think he'd go. That's said to be part of the reason Coach Stallings left, but I can't imagine Alabama hiring anyone that stupid.