Gartner analysts predict Vista will be the last "monolithic" release of Windows. Whether or not their arguments are correct, they do make a good case for a modular Windows that would (as a desired side effect) also make subscription to Software Assurance plans mandatory. This by it's ability to receive timely updates and even upgrades almost automatically.
Between the potential of virtualization technologies and the ever growing complexity of Windows (now said to be well over 50 million l ines of code in Vista) MS is forced to do something. Gartner makes a case for modularity, much like that present in Linux distributions to manage it.