Agreed. All the tech popularizers and business people focus on the non-exclusive license for DOS they gave IBM as the smartest decision Bill Gates ever made, and I guess it was, but for my money a very close second was hiring Dave Cutler and his people from DEC to build a real OS kernel. Without the NT kernel to fall back on, Windows would have collapsed under its own weight long ago.
On the other hand, one of the worse ones was the fiasco involving OS/2 2.0, which was much closer to NT than Win9x was and for example could have been used to legally attack DR-DOS. Why did it take 10 years after Intel released the 80386 before 32-bit programming became popular? I have a thread about it here:
http://www.os2museum.com/wp/?p=516&cpage=1#comment-3413
