I'd probably add that it isn't just Apple in this position, either. PC brands like Apple switching to AMD wholesale aren't just a matter of swapping things out. The supply chain needs to be able to handle it. And generally, an established market with older companies are less likely to stab out on things that introduce risk. They'll let the smaller companies prove things out and take the risks.
So yeah, I agree it's a case of:
- AMD needed to deliver on the promises on Zen 2. These chips were pretty rough around the edges right up through release day with both BIOS and Microcode updates needed (CPUs with a day one patch, almost funny), making the engineering parts hard to evaluate. I decided to build a Ryzen 3600 gaming PC around launch week, and that was not a great idea. I like the machine, but it was a bit of a mess as I was needing both the fixes for Destiny 2, but also some of the weird "let's boost while mostly idle" behavior these chips had at launch. I can totally see Apple, Dell and the like all taking a wait and see approach here.
- AMD needs to be ready to scale up. A couple large PC brands switching would let AMD greatly increase their sales, maybe even double it. That also means they need to be able to deliver those parts. They are wholly dependent on TSMC for fabrication, which also introduces more moving parts for AMD to deal with when expanding production.
- Apple/etc needs to update PCB designs and BOMs as AMD is scaling up. So it makes more sense to wait for the next refresh, rather than change course in the middle of a cycle.
The 2019 Mac Pro in particular, along with the release timeline for the ThreadRipper 3000 chips was just badly timed. Apple would have been taking on the risk of jumping over to CPUs that are relatively aggressive and ambitious, based on engineering samples that weren't really ready for integration into pre-built systems to be stress tested. And trying to launch shortly after the CPUs themselves did. It would have paid off if they did, but it may have made the Mac Pro even more late. I suspect in some ways Apple wanted to cut down on risks like that in the short term.
Now, if Apple were to get a good relationship with AMD where Apple could work closely with them on to get engineering samples faster and with fixes earlier. I could see them jumping over easily. Intel and Apple do share a business relationship that benefits Apple, which is part of the calculus for Apple. Having input on Thunderbolt was good for them, and I can't imagine CPUs like the BGA desktop packages for the 8500/8700 for the 2018 Mini would exist without Apple asking for them.
Going to AMD would impact that relationship, and so from Apple's perspective, there's a decent level of risk involved for them. I'd be surprised if they aren't at least trying to see if they can put an AMD APU or CPU into a Mac at some point in the next year or so.
At the very least, Apple has been making the OS less Intel-centric lately. One reason you can build a Ryzen Hackintosh if you want now.
EDIT: In general though, I do think Intel has a problem. The inertia with the big PC brands will help give them time to respond, but Intel has overly segmented the market in order to maximize profit: Ultrabook, Laptop, Performance Laptop, NUC, Desktop, HEDT, Workstation, Server. These all have product lines super specialized for that segment. But it tends to force people into particular buckets as well. Want ECC? No affordable HEDT CPU for you, pay up for a Workstation CPU. And AMD is taking advantage of how Intel has sliced up the market to maximize profits to wedge themselves into the market. Especially in the Desktop -> HEDT -> Workstation space.