Take an example to 13 years of XP!
I mean; like them or not if you will, but you got to admit that by supporting an operating system for nearly 13 years Microsoft has performed no easy feat. Sure; one reason for all this is because XP has become one of the most popular Window versions so far (though I think 7 is getting there as well).
But even so; to me this is solid proof that there are lots and lots of people who don't like or want to see their desktop, browser, or whole OS in general change on a whim. And I think its this reason why many people will still continue to use Windows over other alternatives.
Computer enthusiasts should keep in mind that not everyone enjoys toying or playing with their computer itself (or its OS). Many simply want to turn it on and do the things they want or need to do; NOW. They dislike being greeted with: "There is a new version of program X, please wait while I perform the update!", only to end up having to wait 5 minutes while all they wanted to do was to do something then and there.
Its this reason alone why I think that the current available Linux distribution's don't have what it takes to be used on the desktop. Sure; when it comes to desktop /functionality/ then there is no doubt what so ever that Linux can cope, don't get me wrong here. The problem is: how long is it going to last?
A real life example: someone who wanted to try something radically different and I installed Debian Linux with KDE and some gizmo's for him. So far, so good. He continued to use it and manually performed some program updates every once in a while. Which at one time rendered his desktop useless, but I don't think you can hold that against Linux because in comparison; Windows has had its shares of issues too.
The real problem came when the next version got out and support for this one ended. At one time I got a call from him; he wanted to install another MP3 player (having heard much good stories about vlc) only to discover that he couldn't install anything anymore because the repository had stopped working.
Luckily for him I knew about "archive.debian.org", though I have hinted that he really should consider upgrading his Debian version. But that is something he doesn't like to hear after having used Linux for only 1 year. As such, don't expect him to become a believer anytime soon, even though he's totally happy with the KDE desktop as is.
I think that as soon as Linux somehow manages to break this cycle and provides longer support then it might indeed be a worthy alternative. Note; and with longer support I don't mean simply continue to supply updates while the process of upgrading from one supported version to the other (Ubuntu LTS for example) starts to become totally impossible without a clean re-install.
But until then.. I think Microsoft is doing a recommendable job by not only coming up with new ideas and setups when it comes to Windows. They also stand by those versions, for a very, VERY long time.
Too bad that a majority of people take it all for granted without realizing the work and effort that goes into this.