What’s funny about this is that Apple is adding a new ARM chip, in addition to an Intel CPU, for better power efficiency. In general, the path to improving battery life in devices is about miniaturisation and simplification; less chips, smaller chips, new processes and architectures for existing chips. Squeeze more transistors into the same space and use the leftover area to pack in battery cells.
With laptops, the tolerances aren’t as punishing. There aren’t the same constraints as an iPhone. Adding a dedicated ARM chip for specific tasks will wring out some extra battery life. Nevertheless, it goes against the grain and doesn’t make sense long-term.
If the main processor was a custom ARM chip too, you could incorporate the low-power magic into it and not need an additional part. Less chips, even better battery life. This is already happening in the iPhone (the A10 Fusion) and has to be the roadmap for the Mac. The technical roadblocks to ditch Intel and x86 will take time, and some points cannot be overcome in advance, but that has to be the target. The goal.
For the sake of completeness, an alternative route would see Apple getting control over x86 chip design and itself creating power efficient silicon that includes all the necessary Power Nap, Touch ID and Touch Bar features. I think this is far less likely because Apple will want to build on its knowledge of producing ARM chips for iPhone and iPad, not start a whole new team for a unique architecture.