DX10 was an absolute failure because it was tied to Vista, which is also an absolute failure with a market share of less than 30%.
DX11 has a slightly better chance of success because although it wont be available for XP, it will atleast work on Vista.
XP currently has a market share of 70%, and if Windows 7 could take 50% of that market share (which would be very suprising as XP users are generally satisfied and unwilling to 'upgrade') we'd be looking at a fairly even split (33%/33%/33%) between XP, Vista and 7. What a mess for MS.

It's hard to imagine any games company would be willing to sacrifice 33% of their audience merely for some extra graphical wizzbangs.
What's more likely is that the majority of vista users will upgrade to 7, and XP users won't, therefore leaving DX11 with a marginal market share increase over DX10 but still no where near DX9's.
Cheers