> still make money from them which likely influences their decision to not release source code. This isn't actually a thing at all. All the id games still sell

> still make money from them which likely influences their decision to not release source code. This isn't actually a thing at all. All the id games still sell like hotcakes moreso now, because of the open source engines that keep those games modern and have dramatically improved over the originals (check out Brutal Doom for an example). Even if your engines and your art assets are liberated, you still do not have a complete game without maps. And while the user community built up around your product would certainly make their own, the fact they are all created on the basis of your game would, almost always, drive dramatically more long-tail sales to just play your original maps that inspired an entire ecosystem than locking up the complete package to rot in video game obscurity like the thousands of Playstation 2 games are doing now.