I would be slightly careful before dismissing XNA so easilly.

Ultimately XNA is an abstraction on top of DirectX9 and DirectX11 - if there are technical reasons why XNA can't work on WP8/WinRT, I suspect the XNA team will overcome them with a new abstraction layer so XNA "just works" for WP8/WinRT.

It would certainly be retarded if Microsoft dropped support for XNA only just as it starts to take off - and particularly whilst it is the only real indie-development platform for XBox360. Arbitrarily preventing Xbox indie-apps from working on Win8 when they have such a dearth of good Win8 apps would be pretty special.