ARM is better for <5W applications when performance is secondary to power consumption, but Intel is beating them at performance/watt ratio everywhere else.
So why not use both? Just like iPhones use a M9 processor for some dedicated computations, why can't MacBooks offload some less intensive computations to an energy efficient ARM processor.
This in no way eats the x86 market but in fact grows the ARM market.
If they bring TouchID to the Mac maybe that is what they'll do as it uses the secure enclave inside the A series processors.
You can't say that, because there aren't 45W and more ARM chips to compare with Intels...