90% of PC games are just console ports these days, hence why DX9 is the shipping backend. Sure you might get updates that provide support for newer versions, but those are few and far between. And remember if anything, DX9 runs faster than new versions before of less features. Just try a DX9 version of a game vs a DX11 one, you'll get a lower fps on average.
Actually no. DX11 / 10 runs faster than DX9 if the game is originally coded for it.
But since the game is originally coded for DX9 (or the 360) and then ported over and then a few additional features added in to make it dx10 compatible is where the performance hit comes from.
Source is a vastly out of date engine anyways, sure it still looks decent but at its very core its still an engine made in 2000 that's just been added on to.
John Carmack (giant openGL fan) has said that DX10 / 11 is better than OpenGL is. And let's not forget, to get OpenGL to the same feature set as DirectX requires a huge amount of extensions (and the effort required to get them to all work nicely with each other).
Windows from 8 on is going to be much different. isn't going to be the same powerful desktop OS that we've grown to love, sure windows 8 still has part of that, but it seems to be going away. You keep speaking of the old windows, and the old linux.
Jesus christ, no it isn't any different. You can use Windows 8 the exact same way you used Windows 7 (minus the start screen). Your beloved x86 / x86-64 apps will still run just as they did before (maybe with a little performance boost if anything). You can still install all the games you want from Steam and they work just fine.
Now if you're on WinRT (the ARM version) then yes all your apps have to come from the Microsoft store but it's not like Steam / Origins will end up on WinRT because face it 100% of their games won't work on ARM.