With TSMC ramping 10nm this year and 7nm next year the choice to move away from intel is logical. TSMC silicon has more features, is easier to design in, and now they have process leadership to and that should give MACs both the OS as well as process leadership over WinTel machines accelerating x86 decline. The benchmark of the iPhone7 already says what the fruity design team can do and when combined with superior process they will crush the Wintel machines.
A long time ago Apple used Motorola 6800 and later switched to PowerPC and then intel seeking superior process. Time to move again as TMG leadership has slipped away. They know well how to migrate OS to new architecture and it isn't like they don't know what to do with their phones.