The problem, Justice, is Microsoft cannot implement change too fast or there is going to be a lot of consumer backlash both from individuals and businesses. In order to move to that next generation of desktop OSes things are going to have to be planned carefully or a lot of people are going to be sitting around with $1,500 worth of Adobe software that won't work with their new OS and something like that would make the rally cries against Vista look tame.
Let's look at the last time Microsoft radically changed Windows. That was when Windows 95 was released and there were a LOT of programs that didn't work with the new OS because they either used outdated Windows API calls or were DOS-based. Yeah, a lot of people still used DOS back then because Windows 3.x wasn't a viable solution for a lot of people.
But I digress...back then there was some outcry and gnashing of teeth, but people made the switch with their apps because there was a lot to gain from doing so. Pre-emptive multi-tasking, improved performance due to software being 32-bit, unified GUI interface, Windows finally being a real full-fledged OS instead of sitting on top of DOS, etc.
If Microsoft wants people to move over/up to The Next Great Thing they are going to need a laundry list of advantages like that or people aren't going to make the move.
And to address your last point, yes you could say that any Windows version after XP are refinements of the same concepts, but only if you are talking about the concept of Windows itself. Once you start talking about the code-base, however, you will find that although Vista and Win7 have very similar GUIs to WinXP a lot of the underlying code under the hood has been rewritten and optimized for performance and security.
Warning: personal anecdote ahead! While the GUIs of Vista and Win7 seem to be very similar to XP on first glance there's a LOT more usability stuffed in nooks and crannies that help you get a lot more out of the user interface than one did with XP. I've been running Win7 since pre-beta builds exclusively and just last week I had to work on a WinXP and it was extremely frustrating. Sure, I could get my tasks done, but I kept looking for the new Win7 shortcuts to doing things and not finding them. It seems like Microsoft is back on track again & I'm really looking forward to seeing what Win8 will bring us in 2 or so years.