Worth noting that Apple has a long history with ARM dating back to the Newton, plus the established low-power advantages it holds.
Considering that Steve went to Intel to discuss the possibilities of them making the iPhone processor, I don’t think independence from Intel was a primary motivator.
Sustainable? I doubt anyone knows. But certainly ARM currently offers a significant value proposition.
ARM honestly has little low power advantage intrinsically speaking as an isa, it's just that most low power cpus tend to be ARM and most arm cpus tend to be low power, same with x86 but for high power and performance.
I have yet to see an x86 ISA that can scale down to the power requirements of ARM. So while the performance/watt might be comparable it seems like getting an x86 design to run at such a low wattage is difficult - otherwise Android x86 would have taken off more successfully.
Considering that Steve went to Intel to discuss the possibilities of them making the iPhone processor, I don’t think independence from Intel was a primary motivator.
Sustainable? I doubt anyone knows. But certainly ARM currently offers a significant value proposition.