It doesn't make any sense for Apple to go for AMD. First off, AMD currently offer inferior IPC and single-threaded performance. As Apple is mostly selling notebooks, they want efficiency and not just throw additional cores to gain additional power.
AMD Ryzen is awesome as it provides much needed competition in the market. Intel has been resting on their laurels for way too long. But for the mobile/notebook market they don't offer anything superior to Intel, so for Apple to go for AMD would only be about economics so they could get even higher margins on the Mac's. Not something that is good for us consumers.
We do not want or need additional cores on systems like these, the only Mac where it would make sense to go for AMD is in the Mac Pro is Apple would opt for the AMD Threadripper aka AMD Ryzen HEDT models that will be revealed this summer. But having Intel on the notebooks, and AMD on the Mac Pro won't happen.
You also have to remember that Apple is tailoring and optimising macOS much more closely with it's hardware compared to Windows. Apple is currently utilising instructions and extensions like AVX and the Ryzen CPU's only supports AVX 128-bit, while Intel supports AVX 256-bit, and will be supporting AVX 512-bit with their upcoming Skylake-X and Coffee Lake line-up of CPU's that will arrive this summer (Skylake-X) and this autumn (Coffee Lake).
So wishful thinking here, if you will indulge me....
First, AMD's single thread performance with the Raven Ridge (i.e., mobile) platform is expected to be on part with an i5-6600. Granted, it does not best Intel (yet), but for most people it will be "good enough." Yuck. However, even with Ryzen, the single-threaded performance has seen gains to put it on par with Intel as software from Windows OS to games have become optimized for Ryzen's new architecture.
Second, AMD's Raven Ridge platform is set to be released around September this year. This puts it on track for Apple's mention of a rumored new MBP project.
Third, while you are correct that Raven Ridge only supports 128-bit registers for AVX (and AVX2), that is per core. Each core can run 128-bit AVX instructions separately or the
two cores can be fused together to perform 256-bit AVX2 instructions, even if it's not quite as efficient as dedicated AVX2 registers. Although this will put AMD APUs at a disadvantage for lengthier workloads, Apple could showcase
benchmarks which show AMD APUs on par or even beating Intel CPUs on shorter workloads. Perhaps the next revision of Ryzen will support 256-bit registers per core, which may then be utilized together for 512-bit AVX support.
Fourth, another unknown factor involves Apple's utilization of AMD's 11-core Vega GPU, which will be an integrated part of the APU.
Again, Intel is still slightly ahead of AMD in terms of performance, and the need to cobble together a solution for 256-bit AVX2 instructions is less than ideal; however, this would not be the first time Apple might be willing to take a step back in order to take a long-term two steps ahead. For example, the transition from 64-bit PowerPC CPUs to Intel's 32-bit Core Duo CPUs saw Apple step back for a generation until Intel had 64-bit CPUs ready. Consequently, I think such a transition to AMD is at least possible.