People don't use WIndows SUA because it's not installed by default. The purpose of writing Windows software is, presumably, so other Windows users can run it, not only so other people with SUA can run it.
Windows is presumably dropping their ancient POSIX support for the same reason they've never supported newer C versions and perhaps for the same reason their commitment to C# was so short lived--because their management team doesn't really care about anything other than the old Win32 API which keeps vendors locked in to the platform.
Supporting POSIX or C isn't going to draw people to the platform, it'll just make everybody else's life easier. Attempt to support newer C++ features is what will help draw and keep developers, as will their typical overhyped promises about an SDK/API revolution in the next Windows product.