Microsoft actually used Windows as graphical shells for "DOS" applications, it was in fact major source of sales for windows before release of Windows 3.0.
Reusing windows 3.x runtime is exactly what you're telling they should have done, except with less reinventing the wheel and being able to reuse all sorts of drivers including for hardware that wasn't exactly ibm pc compatible (like Japanese specific high resolution video)
Reusing windows 3.x runtime is exactly what you're telling they should have done, except with less reinventing the wheel and being able to reuse all sorts of drivers including for hardware that wasn't exactly ibm pc compatible (like Japanese specific high resolution video)