Jason Snell for MacWorld:
If the Mac (and the PC market overall) were a thriving, growing business that was a major part of Apple’s future direction, I would probably be ready to beat the drum for a forthcoming ARM transition. And yet… the Mac is less than 15 percent of Apple’s total business, and its sales are relatively flat in an overall PC market that’s contracting.
I don’t mean to sound the alarm bell about the Mac–I think it’s got a good future ahead of it, and I’m one of those people who believes there will be a bunch of new Mac models in 2017, despite the empty year 2016 has been for the Mac.
But a processor transition is a major undertaking. It requires a huge engineering effort, both in terms of hardware and software. Mac speeds largely start where iPhone and iPad speeds end; Apple will have to push hard to add even more power to their chip designs to support the higher-end applications of the Mac. And every moment Apple’s chip designers are working on new custom Mac chips is a moment they’re not focused on the next generation of iPhone processors.
I think Jason hits the nail on the head here. There are plenty of reasons why Apple might want to make a transition to chips designed in-house. None of them, however, are enough to make Apple want to take resources away from iPhone.