Predictions for WWDC 2013 - Apple announces that OS X 10.9 will be the last Mac-specific OS X, after that
OS X "goes to 11" and to the PC. Apple may decide to certify PC vendors or have some sort of curation mechanism to make sure Apple OS experience is not compromised.
Justifications for the speculation:
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Mac OS X has been renamed to OS X and "Mac" branding removed.
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Apple wants to concentrate on mobile. Intel wants to concentrate on mobile (Haswell is all about mobile). Microsoft wants to concentrate on mobile (Windows 8 is all about mobile). It's 2013 and everybody and their dog is crazy about mobile. Existing Macs will get updates but generally desktop products is NOT how you make investors happy in 2013. Reviving the long-abandoned Mac Pro doesn't make any sense in the current context.
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Apple wants to make their own hardware - CPU, GPU, the whole works. But they want to make
mobile hardware, not desktop. There's not much merit in designing desktop systems in the near future so they'll concentrate on mobile hardware and iOS. The desktop OS will be an extension of iOS so it will be a win-win for Apple.
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Macs are already PCs. Heck, Mountain Lion is flawless on my 6-year old PC (which I didn't even build for OS X). I had more driver trouble with Ubuntu than OS X.
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PC users are increasingly unhappy with Microsoft (Windows 8...). Now is the perfect time to strike.
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As PC users begin to use Apple's OS they will be introduced into the iCloud/iOS/iDevice ecosystem. They will buy software through the Appstore. They will buy more Apple mobile devices because of good integration with the OS.
Pro customers will get their pro Apple OS systems from 3rd party vendors. Apple takes credit for taking a bold step forward. They can also put a spin on this by saying they can now concentrate on mobile devices (should be great for their stock prices).