The reality is that mostly people aren't going to leave Windows, so if Valve and Linux force Windows to improve it's still a win.
While I mostly agree with this, every time I see this mentioned it reminds me that MS-DOS Windows was not very popular, until a Microsoft employee offered to port Doom to DOS Windows, because he saw that if games ran on a platform people would use it and migrate naturally, that employee was called Gabe Newell. So I do have some hope that there's some bigger migration, and in fact we've seen the numbers steadily rising, and these sort of things tend to be exponential, so I wouldn't be surprised if it picks up speed.