People leaving Windows is potentially an existential threat for Steam if the platforms that they go to don't support Steam well. Consoles and iOS are flat out due to non-negotiable platform policies. Android is possible, but tough. Mac OS X is doable, though word is that Apple has historically been tepid to game developers on the platform. I believe that these are the forces that lead Valve to choose instead to invest into linux, and build an OS of their own on top of it.