Could look at this a few different ways.

On one hand, some athletes are simply more durable than others. And some (though not all) science would support that underhand pitching puts less stress on the body than overhand.

At the same time, there is no body that doesn't show wear and tear - at least in the form if diminished performance. So, even if they can throw game after game, it's no doubt that their velocity will decline.

Unfortunately, many (most?) HS coaches aren't looking past the first week of June. They aren't worried about the next season, and certainly aren't taking into account the load that the athlete will carry in the summer and fall.

Probably the most common injury that occurs in situations like what you describe isn't physical... it's the damage that occurs to their recruitment potential!