You're making the incorrect assumption that apple would in fact emulate. As they control the whole stack, they can (relatively) quickly switch architectures. They'd lose the market that does in fact install windows on mac hardware, though that is likely a negligible niche. So for the vast majority of their customers there would not be a performance penalty. Quite the opposite.
Apple chips are in terms of tech already competitive with anything Intel has to offer [1], though of course currently optimised for mobile. Introduce a dedicated chip for macOS without the heat and power restrictions and Apple won't be chained to intel any more. It will save the extra $50-500 they pay intel for every chip. On top of that, the battery will be smaller/cheaper; it will require fewer external chips as the SoC will incorporate this functionality (also cheaper, more control); the retina graphics will be driven by a chip that is adequately powered and optimised (intels graphics chips are extremely slow): all of the sudden the whole package is extremely compelling.
All you lose is windows compatibility. A small downside to multiple big upsides.
Apple chips are in terms of tech already competitive with anything Intel has to offer [1], though of course currently optimised for mobile. Introduce a dedicated chip for macOS without the heat and power restrictions and Apple won't be chained to intel any more. It will save the extra $50-500 they pay intel for every chip. On top of that, the battery will be smaller/cheaper; it will require fewer external chips as the SoC will incorporate this functionality (also cheaper, more control); the retina graphics will be driven by a chip that is adequately powered and optimised (intels graphics chips are extremely slow): all of the sudden the whole package is extremely compelling.
All you lose is windows compatibility. A small downside to multiple big upsides.
[1] http://www.anandtech.com/show/7910/apples-cyclone-microarchi...