You could separate the rendering logic from the game logic and create messaging channels for them... Also, the game specific code is rarely worth anything without the assets... that's why Id has released these codebases, so that people can learn/adapt... the assets retain copyright, etc. Also, you can separate out the game runtime from launcher parts... the launcher can contain other communication channels and be responsible for delivering assets.

As I said, it isn't even about creating a perfect abstraction, just enough to minimize the effect of piracy.

I think when engineers answer these questions it's easy to get bogged down in issues like whether something is a single process linked together or whether you have multiple processes communicating with one another. I don't think these arguments have serious legal merit.

The question at hand is whether the game you are making is a work based on the engine. If true, to comply with the GPL you must release source. Establishing communication channels so that you can put your game logic and the engine in separate processes are not a silver bullet, they don't change the question of whether your game is a work based on the engine, they mostly change the mechanics of how your game is a work based on the engine. There are a lot of difficulties in making this argument.

The court is probably not interested in hearing about whether different components are running in the same process and linked together or whether they are running in separate processes and communicating through IPC. At best, I'd say that you're running the risk of an expensive court case, and at worse I think you stand a good chance of losing it, not having enough money to fight it (because the legal arguments are quite sophisticated), and suffering from things like DMCA takedown notices in the meantime.

I think the trap that software engineers fall into, far too often, is that we try to answer legal questions using the same techniques we use to solve technical questions. The question of, "Is this an executable?" or "Does this use dynamic linking?" is something you can answer based on your definitions of what an executable is. However, legal arguments don't happen in such a mechanical fashion. Contracts and laws are written in prose, not code, and they are interpreted by humans which are basically trying to do understand some combination of "what did the author mean?" and "how would people generally interpret this sentence?"

In either case, the game isn't much use without assets... and those can be copyrighted separately from the executable.

That’s not really true, you can swap out the assets for your own. People do this. You just have to swap out all the assets.

Projects vary between “total conversions”, which give you essentially a new game with the existing engine, to things like Freedoom, which are complete sets of replacement assets for the stock game. Freedoom isn’t really a different game from Doom, but it combines the GPL Doom source with a new set of assets so it’s completely free.