There seems to be a "Catch 22" to being a defensive coach under Saban. The good is that you are going to be working under one of the most respected defensive minds in modern football. I happen to think that if we lost Coach Smart, CNS would find the next Smart, Muschamp, etc. to bring in and begin training. That's not to say that I think Smart is useless; just that our defense wouldn't suffer a loss of braintrust.
And, that is also the problem that I think Coach Steele experienced. Right or wrong, the perception is that Coach Saban is running the defense. As a defensive assistant, it is very difficult to prove or stand on your own merit, because it is not clear how much of the success is yours or is really Coach Saban's. This almost doubles in Smart's case, because Saban is the "GA" coach for Kirby's positional responsibilities too: the secondary.
You wouldn't think that CKS would leave Bama for a lateral title move (i.e., DC @ UGA), but just like Steele, Kirby may start to believe that he will never get quality opportunities for a move up the ladder until he has shown that he can go out and do it on his own.