not completely, because the loss in backwards compatibility would piss off businesses. that's the reason they've been keeping around aging architecture to begin with.
with windows 3.1, 95, and 98 they were consumer versions. around the time of 98 they released NT for businesses, better than the consumer versions but it lost a lot of backwards compatibility because it wasn't based on DOS anymore. consumers got XP later on built on NT.
Vista will probably end up being a transitional OS, like 98, and sometime in the middle of Vista's life they'll release a fully ground up new OS for businesses, and a few years later, when they've managed to get workarounds for a lot of backwards compatibility issues, move consumers to a new Windows based on that new framework.
people always forget that almost everything out there is running windows. self-checkouts and cash registers run windows. businesses would be pissed if suddenly their programs don't work on the newest windows, and that's the problem microsoft has with redesigns.
Apple's OSX was a complete redesign over OS9, and there were serious backwards compatibility issues, but since there weren't enough people to care it was ok for them to do it.