Has it? I think that Microsoft is too busy with its own forced obsolescence initiative to worry about its competitors. After all, it has been trying to kill Windows XP for a few years now as part of it and that has not worked very well. In the mean time,
Mac OS X has been siphoning significant marketshare from Windows and other UNIX-like operating systems have been growing in popularity too, although they have been growing at a slower rate than Mac OS X.
Oh yes. While Apple is busy diverting resources and effort into the iOS devices, Windows 7 is selling seven copies each second.
I'm not saying that Apple is doing badly, au contraire, but Linux is pretty much dead on the desktop and reeling back on the server, and OS/X is dramatically failing to capitalize on the runaway success of the iPhone and iPad.
Vista was a necessary technological step to pave the way for the current Windows generation. And still managed to outsold every other OS on the market.