More likely they won't do it because it would mean that everything made for Win32 would not run on the new platform. This includes all programs made with various other libraries (MFC, WTL, QT, Fox, etc) that "plug into" Win32. Hence they'd have vastly fewer application that would run on this new OS, and people trying it out would complain that "this version of windows breaks my applications".
On the other hand it would be interesting (Singularity comes to mind), but they'd have to call it something other than Windows.
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More likely they won't do it because it would mean that everything made for Win32 would not run on the new platform. This includes all programs made with various other libraries (MFC, WTL, QT, Fox, etc) that "plug into" Win32. Hence they'd have vastly fewer application that would run on this new OS, and people trying it out would complain that "this version of windows breaks my applications".
On the other hand it would be interesting (Singularity comes to mind), but they'd have to call it something other than Windows.
They can avoid program breakage by emulation, much like Apple did with early releases of OSX.