You seem to be overlooking a crucial element in the "OS Wars". As many proponents of Linux or Mac OS seem to do. The issue is consistency and familiarity. These are the primary reasons that MS has stayed on top as long as it has.
First, you may point out that Linux "scales up extremely well, and scales down extremely well." You can also point out that Linux is "free, legal, [and] has no viruses." However, you can't really say it is easier to use than Windows. Not for someone who has never used Linux or any other OS besides Windows.
Whether XP is their first OS or they have progressed through the iterations from Win 3.1, they know Windows. They know where they need to go in the UI to perform the tasks they need to accomplish. They have spent valuable time learning how to perform these tasks in Windows. They are comfortable with it and they become proficient (to some degree) in its use.
Now, one day their Windows is taken away and they are given Ubuntu or Mac OS X. They have no idea where to go to perform the tasks they used to perform with ease. They have to spend their valuable time to relearn a new OS with a new UI. Their actual productivity takes a big hit and they are frustrated that they can't just get their work done. Imagine if that was everyone in your company!! Sure, the company could spend numerous man-hours training the users on the new OS, or they'll have to beef up their IT department to hand hold every user as they painfully become acquainted with the new OS. Or, they can keep using Windows.
Sure, MS makes some minor changes from time to time, but these are done gradually and limited. This gives time for the users to acclimate without making them feel lost. I would love to see MS "wake up real quick with a real, next-generation operating system of the future. It needs to be secure, flexible, low-latency, scalable, modular and customizable." However, look at the backlash they got from the changes they made to the UI in Vista and Office 2007. You can argue about the effectiveness of MS's implementation choices, but the underlying factor is they tried to change Windows for the better (at least that is their intent) and users hated it. I would argue that is one reason Apple and Linux saw an increase in adoption.
I'm still not sure how well Win 7 will go over as it has the same UI as Vista. As I recently moved my Windows box to Win 7 from XP, I can attest to the minor inconvenience of finding where MS has put things in this UI, and I consider myself pretty adept with computers and technology in general. Overall though, I like Win 7 much better so far.
The bottom line: advanced users and computer enthusiasts can adapt to a new and different OS much easier than the average user. Corporations are not going to go through the headache and growing pains of switching all their users to a new OS when many of the users do not know much more about Windows than what is required to accomplish their daily tasks. I believe Windows continues to persist as the dominant OS because MS does not make drastic changes to the OS. If users have to relearn how to use Windows, then they might as well learn another OS. That is when Linux and Apple benefit.