Doesn't it have an API? It could mandate that "random files" should only be created and deleted via the API, and update the manifest accordingly. Put the game in a read-only folder to make sure it happens.

Steam actually sells a number of games which haven't been modified for use with Steam at all - no DRM integration, no achievements. Further to that, it sells games which use closed engines that are never going to be modified to use Steam's APIs to do things.

Or worse, basically just glorified installers for games for windows live, or ubisoft's giant portal thing that you have to then run simultaneously (or in the right order) to get to the game.

Its definitely a rube goldberg machine in action.

I feel like the idea of sandboxing its progeny is going to need to look like docker or some sort of container where it appears to be a standard OS (since games use a lot of low level hacks) but is actually partitioned from the rest of the system.