I think we'll see DX11/11.1 becoming the standard once the next-gen consoles come, with GPUs supporting the DX11 feature sets.
I think the next-gen consoles is part of the reason to why we see more and more DX11 games now, that they want their engines more ready for next-gen the moment the consoles hit the market
This is what we'll see. In the next year or two, most major new games/engines (outside of a few sequels that use a tweaked/updated "old engine") will have DX11 baked right in. Through today, only a few engines make strong use of DX11, and a few only used one or two things.
The next console generation (PS4, next Xbox) will be entirely DX11-based. Most engines will ship with a default of DX11, though some might offer DX10/DX9 downgrades (I suspect DX10 will largely be ignored - DX11 or DX9 will be the options in a menu/config file, if even presented).
Some engines/games will offer DX11.1, but no game is going to be so hardcoded into DX11.1 that certain features cannot be disabled to allow it to run in plain DX11. Even if it's developed from the ground up with DX11.1 in mind, gamers will be able to play them (with reduced visuals) in older versions. At the worst, you'll find games that simply cannot be installed/played without DX11, but nothing will be released in the next four or five years that blocks out DX11 systems in favor of only allowing play with an OS that has DX11.1 or better.
In time, I think DX11 will become the new "DX9". So many games, even today, offer DX9 support as an option. Hell, many are completely coded with only DX9 in mind. DX9 is aging, and the latest OS to have DX9 as the best option is quickly becoming hard to find on systems. Windows XP is not going to be a target platform for anyone, for much longer. I imagine developers are going to realize it's better to just drop support entirely and focus on the more recent market.
I don't see developers just moving from DX9 as "base" to DX10. Vista has a limited user base at this point, though yes, there are still Vista systems out there. I just don't see them fussing about losing Vista systems as well. Drop DX9, but move to DX10 just to keep potential Vista holdouts as a possible customer? Might as well make "the big leap forward" and just move to DX11. Win7 has a larger install base, and while that's officially abandoning XP/Vista, they have to realize these moves would come soon enough.
At this point, anyone still on XP or Vista either a) just doesn't care about computers that much (and thus, aren't in the PC gaming market), or b) needs a swift kick to the nuts. Why not at least be on Windows 7? Both XP and Vista are slower in every way than Windows 7 (and 8), and eventually driver support will dwindle (it might already be happening now, not sure... I have recent hardware all around, and have kept recent with OS installs).