Let Microsoft develop DirectX as they see fit, and target whichever platform they want with it. It's their product and I for one would rather not have things as they were, back in the days of 3DFX Glide and other proprietary API's, or going further back, DOS, where each game developer just did their own thing in terms of hardware support. Furthermore, I would not like to see it forced that new API's must always support older operating systems. The consumer always has a choice. If a person needs the DirectX 11.2 feature set, for whatever reason, it is always available to them.
Think about what you're saying...
DirectX 10 was never made available for XP because of changes made to the way the operating system works. It would be a significant waste of company resources to try and port all of the features of DirectX 10 back to Windows XP, as you are asking to essentially create a patch that updates many of the core operating system components, which in essence would be some sort of grotesque hybrid between Vista and XP, which I'm sure no company wants to have to deal with. If you're not going to update the core OS, you're likely to tank performance, at which point, DirectX 10 on XP would seem rather useless.