It's been a while since I read it, but I think Anthony Wagner made a similar point in his "English Genealogy", that most people with mainly English ancestry are probably descended from Edward III. He also made the point that there has been a good deal more social mobility than people realize, that if you could find all of your ancestors back, say, 6 generations you would probably find a considerable social range. I'm saying this from memory so I can't quote chapter and verse.