I think this is a very subjective viewpoint, which is understandable considering the website we're on, but it still overlooks how huge the PC market really is, along with the Windows userbase, and the nature of a lot of its users. There's a huge segment of the Windows userbase who will always be beginners at computers, and their habits and methods are usually mindblowing to any "knowledgeable" user. One such example was the mass of people who typed "facebook login" into Google any time they wanted to log-in into Facebook, and got thoroughly confused and angry when the first hit suddenly led them to ReadWriteWeb's article on Facebook's log-in, filling the comments section with cries of where is the log-in, what is this crap, shitty re-design, let me in!, etc.

Because of users like this, Microsoft is where it is. Any big revamp, and they'll just get millions of confused customers who suddenly can't make sense of the most basic functionality. It is Microsoft's own doing that they made their users get used to interfaces like this, but it's not as easy as just scraping it and throwing up something new.

It's also easy to forget that Windows has a userbase 10 times its nearest competitor (which is to say this brings in challenges which competitors might not face).