I've been using one trackball or another, usually a Kensington Orbit since they became available, for the majority of three decades; the only times I've had to buy a new one were when I needed more for extra computers or when new technology became available (such as when optical became a thing). I've never had a trackball ball wear out or even show signs of wear.

Actually, that's not 100% true. I did have one Orbit just stop consistently responding to input. The ball was fine, though; in fact, I'm pretty sure I still have the mouse and could just put the ball in a new unit.