Usually you're pretty on with your posts but I think you are ill-informed here. All in all, I think if you looked at the whole arena of guys who had been officially named "Head coach in waiting, " you'd find no consistent indicator that it worked at all to retain coaches or as an indicator of future success as a head coach.
Even the use of the title "Assistant (or Associate ) Head Coach" does not mean "you're the next head coach." It serves as a mechanism to pay someone more money as well as a little expansion of responsibilities. It's like being called a Vice President at a Bank.
As a matter of fact, Mike Elston's title is "Associate Head Coach." but there is little chance he would replace Kelly if he left at the end of the year. And if Kelly couldn't coach on a given day, I suspect Bill Polian would be the substitute head coach.
Re: David Shaw's promotion from the Mercury News
"Bowlsby picked Shaw over a group of finalists that included associate head coach Greg Roman and defensive coordinator Vic Fangio.
The difference was Shaw’s unique combination of attributes: As Stanford’s offensive coordinator the past four years, he provides continuity for the program at a time of unprecedented momentum; as a popular former Stanford player, he brings a deep passion for the school and an understanding of its academic and athletic missions."
As to Riley, there were two other coaches titled Assistant Head Coach and Associate Head Coach. As I remember, there was general surprise in college football when Riley was named Head Coach.