I would've had my doubts, until Apple somehow made ARM competitive with x86. A trick they couldn't pull off with PowerPC.

Yeah. From what I've pieced together, Apple's dropping PowerPC ultimately came down to perf/watt and delays in delivery from IBM of a suitable chip that could be used in a laptop and support 64-bit instructions. x86 beat them to the punch and was MUCH more suitable for laptops.

Interestingly, the mix of a desire for greater vertical integration and chasing perf/watt is likely why they went ARM. With their license, they have a huge amount of flexibility and are able to significantly customize the designs from ARM, letting them optimize in ways that Intel and AMD just wouldn't allow.

I guess linear speed barely ought to matter, these days, since parallelism is an order-of-magnitude improvement, and scales.

It is definitely a complicated picture, when figuring out performance. Lots of potential factors come together to make the whole picture. You've got ops power clock cycle per core, physical size of a core (RISC generally has fewer transistors per core, making them smaller and more even), integrated memory, on-die co-processors, etc. The more that the angry little pixies can do in a smaller area, the less heat is generated and the faster they can reach their destinations.

ARM, being a mature, and customizable RISC arch really should be able to chomp into x86 market share. RISC-V, while younger, has been and to grow an advance at a pace not seen before, to my knowledge, thanks to its open nature. More companies are able to experiment and try novel architectures than under x86 or ARM. The ISA is what's gotten me excited again about hardware and learning how it's made.