This isn't a new decision, it came out a few years ago that you'd end up with disparate instruction sets between the main and efficiency cores, and they got war

This isn't a new decision, it came out a few years ago that you'd end up with disparate instruction sets between the main and efficiency cores, and they got warned it would be a problem. Their disabling of the instructions to avoid it came about as a response. Both Microsoft and Linux devs warned them about the difficulties it'd present.