I agree with you about coaching ability and playing ability not being correlated. However, another reason why there aren't many great former players coaching is that coaching as a career takes a very long time and you have to start early and before you do all those settling down type things people in their 30s typically do. If you have a great NFL career of, say, 10 years you're going to be getting out of the league at around 32. Are you going to want to take an assistant coaching job at Wagner or Kent State or, worse, a high school? Because that is what you're going to have to do. Maybe if you're lucky you'll go to a major conference school, but you'd have to be a GA in all likelihood. Chances are if you've made your millions already (and kept some of it) you're going to lack the same drive, motivation, and flexibility that the younger guys who don't have money and are wanting to buy a new used car or be able to start a family have.