>>3. The game can't install rootkit-like spyware to inspect other processes on my system (hello Warden).
Isn't Warden a mechanism to detect cheating?
Isn't Warden a mechanism to detect cheating?
Absolutely not: Sandboxes keep things in, not out, and cheaters are typically the lawful root-administrators of their machine.
So they just run the games sandboxed and launch their cheats in a more-privileged mode. For example, a cheat can read the game-memory and provide a "radar" overlay to help the user locate hidden enemies. More advanced versions can could replace game-code (from the outside) or even tinker with the network-stream.
The game, meanwhile, would have no recourse, since it would (correctly) be unable to see or influence anything outside of its sandbox.
Also you are technically wrong. Part of the large uproar is that these apps prevent overlays and other features. So they do in fact keep some things out (even currently as implemented).
Then you're just renting your computer from somebody else who can dictate how you use it. ("Saint Peter don't you call me 'cause I can't go: I owe my soul to the Microsoft store...")