Someone who doesn't have to respect copyright law could use it to make their own game like it, saving years of effort. It'd be hard getting up to speed on it, but it could be worth it.
Particularly not when there are properly documented, stable and widely used game engines your devs are probably already familiar with available for no-up front fee.
Compile cyberpunk, stick a couple of new assets and a new name on it and sell ot for $5. I guarantee it would sell
Also code for the game itself means almost nothing without the tooling to produce assets for it. It could be written for scratch but then....just.....use unreal or unity? Pirate copies if you really don't care. Like, I work in games development and in house engines are incredibly inflexible, they are made for one game only usually and it's a titanic amount of effort to use them for anything else. You could probably use the Witcher engine to make a crappy clone of Witcher, but you're not going to use the engine for anything else.
Considering it's closed source I suspect it could get quite far before anyone notices. Also, people will buy legal copies, you over estimate how moral people are. Remember most of us shop on Amazon even tho we know it's terrible. And the money from those copies will go to CD Projekt Red.
Hah, one of the reasons I left the industry was I was frustrated that we were supposed to build the sequel to our game using a different custom engine intended for a different genre of game.