Keep in mind that while you don’t see much 68k outside embedded these days, you still see POWER in supercomputer rankings, and it also appeared in game consoles.
If Apple ends up doing it, the proposition would almost certainly be different and it seems very unlikely it would involve things like running your x86_64 macOS apps on your brand new Mac except three times slower. Thinking about this in terms of previous changes (or in terms of PPC history details) just seems obviously wrongheaded to me.
The Macbook Core i3 is barely enough to run Safari or iTunes and Apple could probably replace the CPU without many of those users ever noticing.
The current A11 chips for iPhones are within 10% of Intel's top mobile chips on Geekbench, and within about 30% on their top desktop CPUs.
It's entirely possible for chip architectures to see 2x-3x speeds when moving from mobile power budgets to desktop power budgets.
An Intel Pentium 4410Y Kaby Lake running at 4.5-6 Watts gets about 1800 single-core on Geekbench, while an Intel Core i7-7700K Kaby Lake running at 115 Watts gets 5600 single-core on Geekbench.
No, they aren't. iPhone X's multithreaded geekbench score is 10k. The 15" macbook pro is 15k. That's a lot more than 10%. It's only close if you look at the lower end Intel chips, the dual core ones (which is what Apple ships in the 13" macbook pro).
What do you think is the most important for 250million desktop users? Because the vast majority of them are sitting idle waiting for interaction tasks, like on your system now.
My dual Xeon E5-2690 v4 regularly loads all its cores and benefits greatly from them, but keep making assumptions by all means.
But if all you want is a chromebook competitor then sure, A11-class works fine. I'm going to guess that the people using Mac Pros tend to care a bit more about just running Chrome/Safari, though. Maybe Apple is just going to completely give up on their historically strong content creation market.