Slightly puzzling

So this raises the question what is mean with 'mandated', because normally you can just run hardware that only supports older DX versions while having newer DX versions installed, vista does run with a DX9 card it just doesn't do DX10 stuff.
So what is meant with 'mandated'? are we talking about the label you can put on your graphics card box to make it say 'window7 compatible'? Is that all? Or is it as you hint about DX11 having no backwards compatibility AT ALL? I think that would be more something to complain about that the opposite like you seem to lament over now.
If it is that then it would be sensible and reasonable to not have it 'mandatory'.

Facts of today are not facts at Win 7 release

I feel compelled to comment on the logic here. While I agree on teh current graphics environment and market share (and the DX9/10 discussion), who is to say that the market, players and technology looks the same in a few years time.

Fact is, we don't know if Win 7 is out in late 2009 or 2011-12. Regardless, just entering 2008 I think it is same to assume that the fgx chip distribution and software market will look different at teh point of the release.

The conservative guess (Win release in 2011-12) would mean DX9 would have to hang around for another 4 years - not likely if you ask me. And on to CPU/GPU development. By then we would probably have the first mobile chips combining nVidia and Physx along with a new generation of Intel integrated graphics (yeah, they are cooking something).

The logic of looking at Win 7 specs and the current market just isn't there...

It doesn't really matter honestly.

DirectX has a limited lifespan as it stands right now anyway. It has always been a fight between DX and OpenGL, in the past DX was a slightly better choice given how poorly the OpenGL API was managed despite basically making you Microsofts whipping boy but now OpenGL is controlled by the graphics chip makers and others in the hardware/entertainment industry so which API do you think hardware and third party drivers will favor?

Now that you've hopefully answered OpenGL consider that OpenGL 3.0 with its 2 extensions will do everything DX10 can but on every OS currently available on the market so which API do you think software makers will prefer? DX10 which only works on Vista and XP assume someone takes the time to crack it since MS is too lazy and/or greedy to do it or OpenGL which allows for cross platform development for Windows XP, Vista, OSX, and Linux. There's also the fact that OpenGL has a full function sub standard, OpenGL ES, designed for portable devices like PDA's and gaming handhelds while DX for Windows CE is fairly neutered as i remember.

Basically when you add everything up DX is on life support regardless of what Microsoft would like to think or how much money they are willing to waste to try to keep it alive.