I'll bet on future WP updates eating whatever perf improvements new hardware will give. Besides, Moore's Law is about the number of transistors. Speed - and especially single threaded speed which is what matters for PHP sites - is not going up nearly as fast anymore.

Yes, but practically speaking, waiting 20 years isn't an option. Moore's Law has stopped delivering rapid single-threaded performance gains and not everything is trivially parallelable.

The fastest single-threaded cpus today are only 2x faster than the fastest single-threaded cpus of a decade ago...