Against Intel's chips that haven't progressed in 8 years, going ARM makes sense. However, AMD's new chips are also leaving Intel behind without needing to rewrite all your x86 software.
Yup, but as I've posted elsewhere, Apple shifting to AMD will leave them vulnerable to what happened with AMD's bulldozer architecture. Or what happened with PowerPC. Or 68k. or 6502.
No.
If Apple move this time, it will be to in-house design so they have full control of their roadmap.
They're going to migrate entirely to ARM in house eventually, it's just a matter of time. This is the first step.
edit:
Actually, it's not the first step at all. Almost their entire product stack is already on in-house ARM.
The Mac is an outlier and is way behind. This is more like the beginning of the last step(s).
Entry MacBook / MacBook Air first. Then MacBook Pros and Mini/iMac shortly thereafter.
The Mac Pro will move eventually too, but intel will have more powerful CPUs at the extreme high end for a while. No more than a few years though I would wager.
Apple are/have already been pushing most bulk number crunching/high end workload off to openCL/metal with dedicated hardware (GPUs, afterburner, etc.). The CPU is becoming less relevant for extreme workloads, and most people should be writing their code to use the processing APIs apple provide - which Apple can port to/optimise for whatever in-house hardware is appropriate.