True, but for example for Win7 they dropped support last year I think. So quite some time.

Yes and no, people can still use win10, it just won’t receive patches anymore. And in this particular case, my best guess is, that most people would rather use and outdated OS for a long time, rather than changing the OS altogether. Not every game is on steam, also not every non game programm is easily available for Linux. Humans are lazy.

OP clearly overestimates how many people would use SteamOS or any other Linux distro for that matter. Most users are casual gamers these days, they are not changing OS just because there is a forced Windows update.