I dont know the particular final cohesion/product/smoothness on each segment of patching DX10 to 11, but neither do you. If each segment blends, or works with the others, calling in the whole team is a moot point. Some are already doing this, tho its prerelease, but others are patching to it , so there must be some ease/low cost benefits involved, or it simply wouldnt be being done.
As to xp, its a dead end, and moving away from every trend in HW we see today. It has very very limited MT support, the 64 bit version is a disaster, it cant support the higher functions we will be seeing in OpenCl very well, and of course DX11. All these things are where were going, despite what the devs want, so again, its moot. If a dev wants to sit on his thumbs and not advance into this direction, its to his downfall, and may have to move from a AAA scenario, to a lower game house IMHLO.
Theres always going to be bottomfeeders, and if they dont change, others will, and it just may be where they end up.
W7/DX11 already has alot of traction, being preceded by Vista and HW, ummmm since they havnt made a DX9 HW since 06. The vast vast majority of pcs sold since middle of 07 have been DX10 compatable etc etc. xp is dying, even in the enterprise sector, withing a year or so, itll happen there as well