Apple's ISA can become more and more divergent from Arm and it wouldn't matter. Apple controls the entire software stack that runs on their hardware.

I think Apple is bound by contract to respect the original ISA (except perhaps the parts they use internally and don't expose external developers to). Forcing external parties to use a translator instead of machine language directly could be one approach to circumvent this.

Also, is not the whole value in using Arm ISA, the ecosystem behind it? If Apple starts diverging from the standard, then it cannot benefit from the existing ecosystem and neither can a software dev say develop on Mac and deploy to graviton. I think it is big lose lose if Apple diverges from standard.

Apple never used any 64-bit cores from Arm. Their own Cyclone was developed in parallel with Arm's Cortex-A57.