How you and your ilk do not grasp this fundamental difference is baffling. When you already have a "good" coach you need to hire a "great" coach for there to be a noticeable upgrade. Thus the majority of reasonable people would like to have an assurance that there is this type of likely upgrade available before they can their good coach. Taking a chance on an unproven coordinator just to make a change is stupid.
Currently there are exactly two candidates who fit that bill: Ken and Andy Reid. Even Ken is debatable.