Look at the competition. They are (1) Linux, and (2) BSD (including Mac OSX). Both are very generic OS, serving a wide variety of settings including the desktop, the server, laptops, and handhelds and at times embedded systems like routers. Both don't care that average user will use only a tiny fraction of the OS. This is general in software: things get generalized to all similar areas. There is no point for MS to create a "WM" from scratch, if MS need one it simply disables the unneeded features from Vista. Because it costs essentially nothing to Microsoft to make a new copy of Vista. Making a "WM", in contrast, means engineering efforts.
Incidentally, I'd call "just disable some feature of Vista" a good enough solution: it is good enough because the average computer has the capability to run Vista anyway (or so MS think), so they don't create a "perfect" solution to match the average desktop usage exactly.