There is a built in catch-22. For someone to be a world class player they will have to have been playing at a pretty high level for many years. That in itself becomes self limiting. A person can only keep up that kind of interest and drive for so long before it becomes too much. Even though theoretically they could have the skills, the mental game and drive to compete goes.
I know someone will bring up some player they claim still plays world class but I can assure you, if you could magically match up that player with a younger version of themselves they are not then player they once were. Maybe they will have flash backs at times but what made them the player they once were will be gone. What it requires to remain a world class player and be able to compete with other world class players will become more weight then an older player can lift after a while.
In fact that younger version of themselves that once would rather play or practice more then eat, now after a few hours of hard play will rather go home and go to bed. That mystery thing that made them great is gone at some point and does not come back.
It shows when you stake one of those older players and all of a sudden you can see it in their face. They don't want to play anymore and wish the guy they are playing would just quit. It is just not all that important anymore.