I think you have this reversed - it's application (game) level backwards compatibility leading to large market sizes of people who have gaming PCs, which leads

I think you have this reversed - it's application (game) level backwards compatibility leading to large market sizes of people who have gaming PCs, which leads to a cycle of dependency which Windows just happens to be a required part. SteamOS, Wine, and similar are attempts to break Windows out of this cycle. Hardware support is similar - vendors go after the biggest market. Also PC gamers are used to installing kernel drivers from random third parties, whereas other OS vendors usually want drivers to be integrated in the main release.