@sun @j i've seen some discussions why more studios haven't done it.

partly it comes down to banks and external dependencies. you might be done with your game but unity isn't, and big businesses tend to buy frameworks and modules which are still commercially relevant even when the game isn't. it's like back when everybody bought speedtree--your daggerfall clone with six monthly active users might be dead but everyone is still buying speedtree.

the other case was dolphin smalltalk that didn't open source for a long time after it died. they owned all the tech, but the debtlords were unwilling to let go until it was thoroughly proven they couldn't sell any aspect of an outdated script interpreter to anyone.