I don't think Win8 in itself is the root of the problem, that has Valve going promoting Linux for PC-gaming.
The problem is that PC-gaming, or at least old PC-gaming, with its paradigms and game-models, has a big enemy in Microsoft.
Microsoft has dropped the ball. They want all videogaming to be console style, with console paradigms. This is because they want to favor their XBox, at the expense of PC-gaming. If PC-gaming is to exist at all, it should play exacty as console games. This is the situation we have today and it has been driven hard by MS to this point. More capable and sophisticated games, running on more capable platforms, are not to exist. That is MS foolish agenda. Foolish because it will in the end cost them what they are dependent on, - the Windows market.
The only way to break out of MS strangle-hold on PC gaming, is to move it to a new platform. Linux is the obvious choice, since it's the most mature and adopted alternative.