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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.