Apple has a long history with ARM, going back to the Newton. Back then (or even in the early iPhone era) they didn't have the time or resources to develop their

Apple has a long history with ARM, going back to the Newton. Back then (or even in the early iPhone era) they didn't have the time or resources to develop their own implementation. They could afford to develop their own ISA now, but why would they? The modern ARM ISA is great. It's hard to picture beating it significantly. And it would introduce another incompatibility, requiring another era of fat binaries.