I wonder how this jibes with Apple's much-derided claims that stuff like XCode had to be $5 due to accounting regulations around free upgrades.

Windows 9 might be free for the same reason -- the new features are in apps and services, and the OS is a compatibility layer than needs to keep up.

For example, hypothetically: Office 2014 is expensive, Win 9 is free to provide access to Office 2014 Viewer or whatever.