The last sentence was the key to that quote. "If they don't have some success, then they will quit before they even get to high school".
If they aren't having some success, then they aren't having fun. If they aren't having fun, then they won't continue to play!!!! That was the point of my statement. Notice also that I mention an AVERAGE pitcher who doesn't throw a curveball will have problems pitching past 12 years old. Of course, there are exceptions. But an average pitcher cannot rely on just a fastball and change up to be successful at that age. They don't have the control that MLB pitchers have. They can't paint the corners, so they will get hit very hard! How long is a kid going to stick with pitching if he can't get anyone out? Remember, I am talking about an average pitcher, not the flame throwers.
I do agree with you on one point. If they rely on the curveball too much at a early age, then they won't develop the fastball that is needed to continue to advance. Keep in mind, I have said many times that a curveball should be the third pitch. I don't like to see pitchers that throw curveballs 50+ percent of the time(as in the Little League World Series).