Well, what I think is there should be a copyright on the exact work itself, but not on derivations thereof. It requires the artist to keep a certain level of performance if they wish to do anything more with it. Essentially, if someone else can follow up on it and do better with it, the right to use it passes then to the other person. However, the artist would retain copyright for the original work. In this case, then, SE would keep a copyright on CT itself, but anyone would be free to make a sequel. And if SE is really that good at making these games, it truly shouldn't matter because their product will out-perform the others, and they should not need any sort of further protection. But if the product is inferior, then the superior product made by others is to be had by the public.