MS has a lot of user inertia, people can't make lateral shifts, they have to make clearly stupendously better shifts, because they don't have time to learn a new mode of doing things.

That's why it kind of surprises me that MS sticks its neck out on UX redesigns like the ribbon or eliminating the desktop, since every change gives the core userbase another excuse to try something new. (Even if there are a few good underlying UX ideas, like larger buttons, more prominent placement of common commands, and saving me a Win key before I just start typing to search...)