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50%!? That would be quite a shift. Windows (and before that DOS) has enjoyed a huge percentage of users for at least two decades. Apple, even now, is just hanging on in the teens percentage range. Most of Apple's customers are iPhone users rather than actual Mac users. Apple recognizes this and is responding in kind.
If you're lucky, Apple won't completely kill their Mac Book Pro line. But I hear the 17in model is definitely on its way out, as well as the Mac Pro tower.
So 50% of PC users running off to a company that might just kill off it's own line of PCs... hmmm probably not. Edited 2012-05-15 11:29 UTC
More importantly, it's unlikely that Apple will hold 50% of the desktop market due to their business model. They earn insane profit margins instead. (Not a bad business model, mind you, but not the volume story.)
Windows has dominated the desktop because any manufacturer can build Windows desktops. Android has taken 50% of the smartphone market with the same strategy.
So a viable challenger for 50% of the desktop market will likely be one that also allows anyone to use its OS to build products - i.e., not Apple.