The performance per watt point is a more than fair argument. It also cuts both ways. If Apple decides ARM is still the performance per watt leader in 20 years, there's a good chance they'll still be using it. If any other processor tech reaches a better spot, they have the expertise and the semi-closed ecosystem to switch processors quicker than pretty much anyone else in most of the spaces into which they sell. It's something they've not only done multiple times, but done well and are known for doing well.