It’s a lot more than that. SteamOS isn’t just Steam Big Picture Mode. There’s some special sauce in there to capture the active window so that you never lose focus of the game window and such. If you just run a Windows machine set to boot into Steam Big Picture on startup, you’ll find lots of times that you have to break out a keyboard to Alt+Tab for a variety of reasons that SteamOS never encounters. And given the other problems Windows has introduced over the past decade, that’s the least of their problems now.