I don't think Microsoft makes much money off of shrinkwrap copies of Windows. Almost everyone I know just sticks with the version of Windows that came with the computer, and when they switch to a newer version it's because they bought a new computer that comes with it preinstalled.
The probable advantage to Microsoft is that they have a less disjointed platform to support. For example apps released for the app store don't run on Windows 7 and earlier, and apps that work on Windows 7 and earlier can't be put on the app store. That sort of thing has been an enormous success killer for some of Microsoft's newer business initiatives, and that in turn means it's a threat to the long-term success of the company.
It might also save on development costs. 3-year release cycles mean you're at great risk of sinking tons and tons of resources into gold plating ideas that ultimately bomb on the market. This new approach means they can fail fast, which means they should be able to produce value more efficiently.