Just imagine where Microsoft and the rest of the IT world would be if they pulled a System 7 and an OSX trick on all of their users. Forgive me if I have the version wrong, but Apple, admittedly for a damn good reason, cut off support for all of their prior OS users in one swell foop. *POOF* The old version won't work. They also did it when they dumped PowerPC for Intel, although it's not dead until now with Lion.
If Microsoft had completely rewritten Windows Vista to be true 64 bit and not compatible with prior Windows versions, or more acceptable, had dumped the old DOS memory structure and all the legacy crap when moving to Windows 2000, to set a time, they would have a much more stable OS and not a lot of the legacy BS issues they've had over time. However to just say that with this version, nothing you have used before will work with it (sounds like Windows Millennium Edition!?) would have probably resulted in many users defecting to Linux and the Mac which would have made things interesting for sure... Eventually Windows, and the Macintosh, have had to 'kill the father', and move on to stability and more powerful and capable systems.