Why would Apple need x86? They control the software stack, no reason not to begin a transition to ARM, unless AMD is significantly stronger in performance/watt than Apple's in-house chips. Even buying AMD, it'd probably be preferably for Apple to transition to ARM.
AMD's gpus might be worthwhile though, especially if Apple's in house GPU isn't all that great or can't scale up to PCs. AMD's cost looks almost like a steal just to get a lock in on Radeon graphics and burn the rest of the company.
While they have done migrations in the past, it was with reluctance and for obvious reasons. Moving from 68k to PowerPC was obvious as Motorola had given up on the architecture. And the PowerPC was a great deal faster. Enough faster that running 68k code in emulation worked very well.
Going from PowerPC to x86 was also obvious. IBM was unwilling to work on performance, and performance per watt for their consumer grade CPU's. You can't put a 140 watt CPU into a laptop. At the time the G5 in particular came out, it was the fastest CPU you could get on the consumer side. But, IBM tossed their roadmap out, and fell behind by a lot. And the G5 had become very power hungry. When you have to have liquid cooling as air just isn't enough, then you have a power consumption problem.
So, going to x86 made sense. Indeed, it was pretty much mandatory unless they wanted to give up the laptop market. Not going to happen, as it's a major profit center.
The Core architecture was fast enough that they could run the PowerPC code in emulation as well. That and fat binaries made the transition fairly painless.
To transition to ARM just doesn't fit what's going on. Their own ARM CPU's would need to have 40 to 50 percent higher IPC than Intel to make emulation palatable. And that ain't gonna happen any time soon.
Further, while Intel has botched a few delivery dates they have worked with Apple, and made them the parts they want. The blinding exception being the SOC's for the iPhone, iPad etc.
There just isn't any compelling reason to transition to ARM, and lots of reasons not to.