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Microsoft is a huge company. They can afford to create Vista's and Metro's that will ultimately become refined. Whey they have to do it in the most offensive and expensive way possible? They have a lot of cash lying around.
That's the way everyone does it. Gnome? redesigns the entire DE, changes the whole damn thing, and they still working on getting it right. KDE? Took them until 4.4 to get really useful. Unity? Holy crap, the things an unholy amalgam of OS X and Gnome 3. OS X? it took OS X until 10.2 to be truly usable, ask any OS X user who used 10.0 and 10.1.
That's the way things are done these days, MS is not the first, or the worst offender.
The old desktop is there, and Metro can be mostly ignored if you want. IT pretty much has to be right now, as there are no killer apps for it.
You mention the interface changes with Linux but you didn't mention that the user has the ability to choose alternative UIs that are closer to earlier versions of Gnome and KDE (MATE, Cinnamon, LXDE, Xfce, etc.) as well as the new UIs (Gnome 3.x, KDE 4.x, Unity). With Windows, you have three choices: Microsoft's way (Metro), remain with an earlier version of Windows, or 'take a hike'. I love Windows but am beginning to become frustrated that Microsoft refuses to listen to those that want the option to retain the Desktop UI with 'START' button (turning off Metro completely). I'm wondering just how bad their 'wake-up call' will be when Windows 8 Metro is released into the wild!
The bigger you are the harder you fall. Don't think something is too big to fail because it is not true. The cost of maintaining such a large company is enormous when the cash stops coming in they will need to shrink very quickly.