You haven't spent a lot of time around coaches is my guess.
What you said is logical and makes all the sense in the world. He is also going to make great make money in the studio or booth if he wants to.
Then the competitive tick is going to return, his wife will get sick of him being home too much, and some big time program is going show up to his door and say "here's $48 million for four years on the books, another $2 -$3 million per off the books".
I remember Jim Calhoun saying he was done. I remember Mack Brown saying he was done. Hell, I remember Urban retiring once before. Coaches coach. That is what they do. He is only 57. I give it 3 years, 4 max. He can take 5 years off and he would be younger than Saban is now. But maybe you are right.