DirectX 10 is not going to be ported back. There are a lot of changes in the OS itself to make things possible in it. This has been stated by the team developing it several times already.
There are countless reasons to go Vista. Since it was developed for so long, the features and improvements just piled up

If you are using Windows, you will anyways. It is a matter of time.
The same discussions were held for XP. Now 5 years later, just 2-3% of people use Windows 98 or older.