Think I kinda know what u mean.
Many people still use XP for a reason relating to their favorite game/game co. still don't know what dx10 is all about.
Too much dependancy on game-engines to be the backbone of the games being created. If some1 like Unreal decided to keep their dx10 engine in-house and lease out dx9 only still, the situation would be that much worse.
Even tho there are many using XP, dx10 and Vista have almost breezed their way in to being a standard (prior to Win7 ofc). U buy new gear, u go for dx10/Vista just because it's the latest thing to that point.
Next year or so, as u say, dx11/Win7 will be the latest thing when a new gear purchase is made.
I'm not so sure it would be on the basis that all the games coming out are dx11 tho.
I can almost guarantee u that next year, game installs will still be asking u to install dx9 hooks.
Buying dx11 as it comes out, just cos it's come out.. not necessarily a smart move - unless as I say ur buying new gear anyway.