> It's similar to asking if people could write a portability layer atop Win32 to support Win16 to support old DOS system calls--anyone can do a subset of that and complain that "Hey, it's easy!" but to do it right (and you must do it right, or else somebody else will complain!) is very nontrivial.

Didn't Microsoft actually do that? Isn't that how we have Win16 support in 32-bit Windows 7 today?