In general, as time has gone on, the number of paid OSes has dropped. From BeOS, AmigaOS, and OS/2, as MS Windows proliferation killed off the aspirants to the throne, they also had a side effect- that there were more free OSes, and less paid ones, as it became clear that Microsoft was the only OS that could make money off of this basic functionality. The only other paid OS out there for a long time has been OS X.
Then, with Lion, they moved to a digital distribution, and with Mountain Lion, they dropped the price to $19.99. Now, OS X 10.9 Mavericks has been released - for free.
This leaves MS as the only paid OS provider, and they have been making missteps left and right it seems.
Are the days of MS using Windows for a large revenue sink over? Or will they prevail against the winds of change, continuing on as they have been?
As an aside, it is very apparent also that this is where the walled garden gets us- the vertical integration path where the profits can be made from other parts of the revenue stream rather than an up front cost.