While the compatibility certainly eased the transition path for Apple, I think the main differentiator between PowerPC Macs and the Apple-ARM iPhones is the num

While the compatibility certainly eased the transition path for Apple, I think the main differentiator between PowerPC Macs and the Apple-ARM iPhones is the number of units. The PowerPC machines never reached the unit count allowing for a sustainable development budget to keep up with Intels x86 investments. With the iPhone, Apple has the production numbers (and especially the revenue) to match or exceed the development budget of any competitor in the mobile phone space. It also helps a lot, that currently the best processes are available from independant fabs, which was not the case in the past.