What will drive any Apple decision to move to AMD is MOBILE (notebooks - they're totally happy with ARM on iOS, as they should be).
Apple's sales breakdown is 80% notebook, 20% desktop. I'm almost sure that each major notebook model (16" Pro, 13"Pro, Air) independently outsells all desktop Macs combined.
AMD has nothing that would fit in the Retina MacBook Air, much less a MacBook refresh.
AMD has an inferior mobile APU (with markedly superior GPU) that would work for the 13" Pro - it would be a CPU performance downgrade, but the GPU upgrade is possibly worth it. However, note that the Surface Laptop (the only premium Ryzen notebook) isn't getting great reviews - people are going to Microsoft's Business store to get Ice Lake instead. A Zen 2 version of the low-power AMD APUs is due next year, and that should be a better choice.
AMD has literally nothing that would work for the 16" MBP. How would pros feel if the fire-breathing 8 core notebook workstation with 64 GB of RAM and an 8 TB drive that has left them feeling heard was snatched away and replaced by a low-power quad core with most of its power in an integrated GPU?
Yes, it's theoretically possible to run one of the Ryzen APUs with a discrete GPU, but why would you want to? Their strongest feature is the integrated GPU (perfect for the 13" MBP, not for the 16")
Even while the 15" MBP was drawing complaints for butterfly keyboards and old Polaris GPUs, CPU performance marched on. The 2016 was disappointing - just like a 2015, 2017 got a surprising performance boost, 2018 was significantly faster (extra cores) and could take more RAM (about time), but had initial cooling problems, 2019 was very fast. Now that there's a new design with excellent initial reviews, a 50% performance hit isn't what Apple needs.
On the desktop side, new Ryzens are mostly a very good fit, assuming the iMac Pro can cool a Threadripper. If not, there's not enough CPU differentiation to maintain the iMac Pro as a separate product (it becomes a high-end CTO of the 27" iMac with a Ryzen 9 3950X, which might be OK) - only the Mac Pro gets Threadripper in that case.
The other issue on the desktop side is the supply problems with the 3900X and the delayed launch of the 3950X. If you assume that 3900X and/or 3950X are maybe available in iMac Pro quantities, but not in mainstream 27" iMac quantities, the top chip on the 27" iMac becomes a Ryzen 7 3700X, which is about equivalent to a an i9-9900K. No big loss, no big gain. There are some gains lower in the line, but the big gains are to two exotic systems that make up a small fraction of Apple's sales.
The iMac Pro gets much better price/performance (and probably becomes a high-end CTO iMac) It gets a 12-core Ryzen 9 3900X as its base CPU, with a $400 upgrade to a 3950X. It only gains a little speed compared to similar core count Xeon-W configurations, but the 12 core comes in at the price of the 8 core base model, and the 16 core costs like the existing 10 core, but performs like the 18 core.
The Mac Pro gets much better chips. Instead of being 8 to 28 cores, it starts at its current base price with a 24 core Threadripper, has a $1000 upgrade to 32 cores, a $2000 upgrade to 48 cores and a $3000 upgrade to 64 cores...
The problem is that moving to AMD would eliminate 5 million or more MacBook Airs per year, cripple 5 million or so 16" MacBook Pros and really benefit 100,000(?) Mac Pros and a million or less high-end iMacs/iMac Pros.
If Apple were to move the MacBook Air and maybe some Minis to ARM, everything that can to AMD, and they were willing to accept two architectures (ARM and AMD), the only remaining problem would be the 16" MacBook Pro. Do they keep one machine on Intel? Can they convince AMD to make some special chips?