Programmer Humor

Yeah that's basically why I didn't pull it out as an option in the first place, it's not always practical. A lot of your proprietary code is going to be external dependencies linked/built against, or your own IP reused from the last project. But not all of it, and I can definitely see that smaller chunk causing a lot of problems.

You need a team that does a lot of dependency management and similar things well while building it, that don't actually help them get the game out faster, to keep the problem manageable. Or a team who specialize in open sourcing games like this, which could become a thing if this was more commonplace.