The only reason the engine can be free nowadays is that it has near zero commercial value to the IP owners. If the lesson is that for a libre game to survive and prosper at the mass market level being worthless is an important enabling factor, that’s not a great conclusion to come to.

Anyway Doom has this great community around it not just because the tools are free, thought that’s a an important enabling factor, but due to its cultural significance to gamers and games developers.