Considering how easy it is to emulate Graphic Adventures thanks to ScummVM, I doubt releasing the source code would impact the sales in any shape or form. People that want to play for free already have the easiest life possible. I think it's one of those cases where emulating is as much convenient as grabbing a copy on Steam.

You rise a good point. After X years releasing the code of many things would benefit the public so much without hurting at all the owners, are there movements about this concept?