If you can't call those "OS"s, than what really is an OS?
I guess it's turtles all the way down.
> almost every serious game is a complete more-or-less real time operating system
Games tend to have very different requirements and different demands from the OS; they tend to regulate the OS to being (mostly) a hardware abstraction layer. In the late 1990s and early 2000s, it was a lot harder to run games on Windows NT versus Windows 9x because DOS was a lot better at getting out of the way of games.