This seems to assume that after black belt, there is no more to learn (except in your method, the ability to teach). But in many styles and schools there is a lot more to learn after black. One may go so far to say that black belt is the start of real learning. While one's physical technique may be excellent at this point, it takes years more to develop the subtle variations of hip movement merging with the hand and the flow of energy throughout the body, and other hard to describe attributes that add to one's ability. More years to learn the advanced applications of kata, use of weapons, etc. In my style (in a good school) this learning process continues until 4th or 5th degree black. After that, degrees acknowledge years of contribution to the style and upholding high ideals. Some schools may not have much depth after black, but in others there is a lot more to learn. I didn't really know and (partially) understand my wife until being married for ten years. We were in love after three years of meeting (that's the 1st degree black) but real bonding and deep knowing took years. As the song goes, "Time can do so much."