I'm more
Actually for low power stuff, ARM is very competitive. I know Geekbench is not a great cross platform test, but nonetheless it does illustrate how good ARM is these days:
4259 / 10713 - iPhone 8 Plus (A11)
4467 / 8520 - MacBook Core i7 (7Y75)
3919 / 7493 - MacBook Air Core i7 (5650U)
Again, I don't believe an ARM MacBook or MacBook Air will appear in 2018, but if Apple were to do this, it would make the most sense if they did it with their ultrabooks.
Of course I am aware of these benchmark results, however, real usage is not a benchmark. The real usage depends much more on specific things such as video-accelerator circuits , supported instructions such as SSE4.1, 5, AVX, VT-x etc. The issue is that it is hard to benchmark that and most people do not take in into account. I do not see a good reason why Apple should create ARM Mac. They already have it - iPad Pro with keyboard.