Do you actually own AirPods or just assume that's the case? Nobody I know has really had to replace their AirPods unless they lose them, forget them in a pocket and wash them, or decide to upgrade after a few years (myself included). Certainly there are some people on the edges who replace them more often for whatever reason, but you might consider the possibility that they sell well because many people (myself again included) believe they are a pretty damn good product.