Cheap and power efficient is mostly not about the ISA. ARMv8 is designed for efficient fast CPUs and it's good at it, but you could do it with x86 if you tried. Intel doesn't try because they want to make expensive high-power server chips.

Although their latest plan is to turn it into ARMv8 anyway: https://www.intel.com/content/www/us/en/developer/articles/t...

The main problem with x86 is that it has poor security, not poor performance.