The game might not be over at the end
When I watched this, the very last scene --with Deborah Unger getting into the car and Michael Douglas striking a conversation with her, and then the camera fading out-- felt like a question mark. Is the game really over at that point? For some reason nobody else considers this possibility when discussing the film. I think it's perhaps because the emotional climax of the film, with the party and suicide attempt, has already happened, and an ongoing game wouldn't appear to have anywhere to go, narratively, or any reason to exist anymore. But I think that last scene could potentially lead to more outrageous scenarios, on and on, forever. Obviously, there would be no logical reason for anybody to want to do that to Michael Douglas, but the film goes down the rabbit hole anyway. It's built on an absurd situation, so having the story "end" with the game never ending makes sense. It would be taking the premise to its "logical" conclusion.
As an aside, I have no issues of any kind with supposed plot holes in this film. As I said, it's meant to be a thoroughly absurd situation. In a (very) good way.