The smart thing to do would be to have Saban and the AD pick a successor about 3 years ahead of Saban's planned retirement. Then get that person prepared to take over when the time comes.
What usually happens, is the super-successful leader (coach, CEO, etc) never really wants to pick a retirement date, stays on a few years too long and things start to go pear shaped. Then they get pushed out a bit in a hurry without a good successor named, or without that person having quite enough time to be ready to take over. And at that point, it often (not always) goes to pot and falls apart completely.
Happens in business all the time. Coaching too. Snyder, Fry, Bowden, JoePa all stayed too long, just off top of my head.