You're asking an unanswerable question about the an individual role in a dynamic organization. You want a general conclusion to a hypothesis about an individual's title in a particular field of organizations and why changes are made in that position even though the individuals are unique and the organizational situations are diverse. Your question is the equivalent of "Why do corporations fire executive staff?" or "Why is there turnover amongst finish carpenters?"
Even legendary coaches like Landry, Shula, Noll, etc. (add Joe Pa, Bear Bryant, Bobby Bowden if you want) seem to run out of gas at some point. Does that mean they were never really "good"? There is no general conclusion because every situation is unique and different.
Situations change. Talent does play a part, obviously, in a football team's success and failure. Still, if you take the Don Shula example, he had a Hall-of-Fame QB in Dan Marino and collected All-Pros in south Florida like it was a fire sale, and yet he still never got over the hump and won a Super Bowl towards the end. So there is a counterexample that pops the "it's all QB" simplistic balloon.