That part about partners is important.
Apple's being a bit disingenuous by implying that OS X Mavericks is completely free. You still need to buy a Mac in order to qualify for those free upgrades, and the true cost of OS X development is still rolled into the cost of Apple's hardware. ("Apple tax," anyone?) It's just obscured because you're buying the complete package, rather than stand-alone software.
The same is true when you're buying a Windows desktop or laptop. The vast majority of Windows computers are sold to consumers as premade, preloaded PCs from manufacturers (sometimes called OEMs) like Lenovo or Dell. Those manufacturers buy Windows licenses en masse and--along with volume corporate customers--account for the majority of Microsoft's Windows revenue.
The only Average Joes who need to actually buy Windows are end buyers, like DIYers with cobbled-together computers, or people actively seeking to upgrade from an older operating system release. Consider it a component cost: You're buying separately an item that's usually rolled into the cost of a computer.
By continuing to charge OEMs and end buyers for Windows licenses while giving away upgrades for free to existing consumer (not corporate) users, Microsoft can essentially have its cake and eat it too. The no-cost nature of Windows 8.1 hints that Microsoft's already heading in this direction. (Again: Apple's "We're going to revolutionize pricing!" cry isn't quite as revolutionary as claimed.)
The point of no return
But if Microsoft stands to gain as much as Apple by giving away free operating system upgrades to consumers, why is Windows 8.1 available only to Windows 8 adoptees? A Macworld coworker of mine likened OS X Mavericks as the equivalent of Microsoft allowing all Windows users from XP on up to upgrade for free.
That's not quite true.
Apple's giving only users of OS X Snow Leopard and above the free upgrade to OS X Mavericks. Sure, that's three OS generations back--just as Windows XP is to Windows 8--but Snow Leopard was released in 2009, the same year as Windows 7. Windows XP launched in 2001.
The hardware world has changed drastically over the past decade and a half, and given Microsoft's hands-off history with the nuts and bolts of PCs, handing Windows 8.1 to all Windows users is simply unrealistic from a compatibility standpoint. Apple, on the other hand, controls Mac hardware with an iron fist, and the high-end hardware it dabbles in ensures 2009-era components still function just fine today.
OS X Snow Leopard was also a key cut-off point for Apple: That release removed support for Macs running on older PowerPC processors.
Windows 8 could be the Snow Leopard-esque line in the sand for Microsoft. While the hardware requirements for the OS are largely the same as those for Windows 7 and Vista, Windows 8 demands little extras, such as CPU support for physical address extensions, NX bit, and streaming SIMD extensions 2, and DirectX 9-compatible graphics--things that ensure Windows 8 plays nice with Windows RT, Windows Phone, and Xbox in the cross-platform, Live Tiled "One Microsoft" of the future.
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