From the article: "News of his donation sent GoPro shares tumbling as much as 14 percent the next day, as investors interpreted the move as a lack of confidence in the stock."

It's not about wanting the stock to do poorly; as the chief executive, he knows where he plans to go with the company, and if he doesn't see it going far, then at least he can save some money. The fact that investors interpreted the move this way indicates that it's not about hindsight, but a poor forecast.

Your point about values rising on average is a good one though - it means that though some of the rich may be getting away with more money, overall donations are still going to balance that. There are still some other issues though.