I don't understand what your point is, coaches are precisely an example of a boss having deep understanding of the work being done, as opposed to being hired purely for his people-management skills. You don't poach the manager of a retail chain to be your basketball coach.

Obviously a coach can't replace a player on the field, that's primarily because playing is more important and pays better than coaching at the top level, so no sports star wants to be "promoted" to a coach, they will coach once they can no longer play. And even then, being a coach is not necessarily the life goal for super-rich star athletes. It's a downgrade from what they used to be in their prime.