Yee, you're mot going to be hurt by open sourcing your game 5 or 10 years later. By that time practically nobody will buy your game anymore. And of the ones who still will,.they likely aren't the ones that would even bother with looking for alternatives other than a big sale on a store page

But then, open sourcing adds to human culture, it lets others modify the game, or use it as a foundation for something new. And those things will credit you, and you will still get some extra benefit/good pr.

It's just a good thing to do, imo.