Consoles and iOS/Android are by far the dominant gaming OSes. PC gaming isn't even all that huge slice of video gaming, let alone computer usage as a whole.
It's good to keep in mind that games like League of Legends alone have over 100 million monthly players. While I agree that mobile is far bigger (especially thanks to Asian countries), PC's slice is too big to ignore. And just going by Steam numbers, seems to be actively growing each year.