I generally agree with this. How many top performers in any field teach versus perform? (during when they are at their top performance level?)
From what i have observed, it is generally not true. In my experience the very best in every profession are, in general, concentrated in the top colleges and universities. In some cases we have weak elementary and secondary school systems, where perhaps it is partly true that those who can't do teach, but i would think that in general, and most particularly, at the university and college level this saying is false. I think there is ample evidence of this. Throughout the twentieth century the US, in particular, has depended on its colleges and universities for major advances in science, engineering, medicine, and the humanities. The lone exception might be the arts, but i'm not so sure of that. Even those developments that we associate with Corporate America, silicon valley comes to mind, very often have an important university connection. It may not be correct to associate "not doing" with "not able to."
Seems the only one's defending teaching rather than being able to do are the teachers!! No surprise there!! Let's suppose you are on trial for your life. Would you retain a lawyer to defend you that has 30 plus years in criminal law with a very high success rate, or a law professor who has never seen the inside of a court room? How about if you need open heart surgery? You want a doctor with years of experience performing the same operation again and again, day in and day out with high success, or a University Professor doing the procedure? And guess what! Civilians who wish to obtain a commercial pilot position usually teach flying for several years before they are even considered for the most elementary flying positions. Do you want Warren Buffet to personally manage your portfolio, or the local college professor who teaches finance? As for trading, how many people who have made millions in the markets are now teaching versus those who have blown up and are now posers selling an "education" in how to trade the markets? As a rule, those who cannot make it in the real world for whatever reason, usually resort to either teaching in some form or find employment with the government. Your question, in essence, is a stereotype. And as painful as stereotyping may or not be, as a famous philosopher once said, historically, stereotypes typically prove to have a loud ring of truth within them.