Right. To me it's unthinkable given the amount of low-level code shared between iOS and macOS that macOS hasn't been running on ARM since day 1 of iOS.

Sure, but don't discount how every specific instance can be a bit different either. As I said they've got expertise, they've got capital, and there are even previous paths to follow here. But at the same time every time has its own unique hurdles. Previously with 68k -> PPC and then PPC -> x86 for example they were going to something that was not merely just an improvement in some important respect right off but also had a clear long very steep growth ramp ahead thanks to fabrication improvements if nothing else. In the PC world we were still very much in either a very steep or at least steeper part of the S-curve. But those days are just plain done, the issues presented by physics and the geometry sizes being worked with now are simply fundamentally harder. There is certainly more room for improvement year to year for a long while, and more chances to grow horizontally with valuable new features, but it's not like a system made now will be obsolete in 3 years either.

Additionally those were coming at points in Apple's life with a dramatically smaller installed base, and they were also going away from something more proprietary (at least in principle, obviously CHRP never actually worked out that well). The move to x86 which brought Macs in line with everything else meant a huge amount of software opened up to more trivial porting, huge amounts more opened up to trivial virtualization, a vast 3rd party hardware market became more easily accessible, etc. That definitely helped offset some of the old Mac software that ultimately didn't make it, even more so because due to above it is still quite possible to run old Mac software fine: Classic can be emulated, and 10.6 can be run under virtualization still which in turn grants access to Rosetta even on new Macs, and the absolute performance advantages vs 12+ year old systems are significant enough that even with the overhead it's still fine.

Basically there are a lot of subtle day-to-day advantages that come from everything running the same instruction set underneath, or at least being able to stick some sort of translation layer in there. Again, absolutely not saying it's something Apple can't tackle, just that it's a big challenge and I think it's bigger now then it was any time previously. Of course, Apple too is bigger now then any time previously! They're not infallible though and I hope they get the balance right here.