So what you're saying is that IW had a commercial license for IdTech3 and made several proprietary changes to it and Id kept getting royalties. I'm not privy t

So what you're saying is that IW had a commercial license for IdTech3 and made several proprietary changes to it and Id kept getting royalties. I'm not privy to the details of Id's licensing contracts, but I imagine that they priced their licenses in such a way that licensing was more attractive than creating your own engine. If IdTech2 was available under a BSD/MIT license then IW could've just taken that, improved it, kept their changes to themselves and not pay Id any royalties. I see your point, but I'm still inclined to think that it wouldn't have made much of a difference to Id's engine licensing business at the time. In our hypothetical situation where IdTech2 was available under MIT/BSD then IW would either be stuck with inferior graphics or would've had to spend significant resources to get it up to the standard of IdTech3 (which also comes with the business risk of whether you'll be able to pull it off and how long it would take). They would also have to rely on themselves to get support, rather than Id. I think executives would in the end just see licensing IdTech3 as the cheaper and less risky option. I think this is what Carmack's tweet is about: At the time his business partners thought that the GPL license would protect their business and insisted upon it, but now with the benefit of hindsight he recons that the code would've had a greater impact if it was made available under a more permissive license.