I could be wrong here, as I have no vast knowledge about the graphics hardware. But I thought currently, API's like DX and OpenGL communicate with the GPU directly rather than with a software source, which makes programming rather static. An API on a 'new' software based GPU should be proven much more dynamic. So if the case happens that DirectX is stepping over on such an API, it would be an entirely new product.

So that's probably why DX10 is only seemingly upgraded with very minor steps. Behind the scenes they did much more to assure a backwards-compatibility safe future. Aside DX10 I bet they were already working on software based API solutions too.