Microsoft invested HUGELY in the core of this operating system and they are not about to pull the plug on it. This is why the next operating system is based on Vista internals.
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Which in turn is still based on the NT kernel, with some added 'features'
The problem with Vista is, it changes the way we are used to dealing with things, and some people don't like change (and all the added extra crap that MS decides to force on people, causing the OS to become a resource hog)
And everyday users aren't going to know how to change a lot of that stuff.
I have not upgraded yet, as I see no reason to. It doesn't offer me any incentive. Not saying I won't...but not yet