I'm an idiot when it comes to chips, but the one thing I thought I knew was that ARM CPUs run considerably cooler and more efficient in terms of performance per watt, especially compared to Intel. Isn't that why they're in every mobile device?

It's historical. In the 80's when an extra 10,000 transistors for decode could be 20% of the chip, ARM had an efficiency advantage. So it got used in certain markets. Which meant chips were designed for that market. So because there were chips specifically designed for the market, they were used. Feedback loop, essentially.

Your explanation is great except for the beginning of the story: in the 80s and early 90s, CISC cores didn't have a CISC to RISC decoder frontend - they implemented complex instructions directly (though with the help of microcode). The size penalty of that was more like 300%, and it was slower, too(!). RISC was a really big deal at the time. It was even mentioned in the, ahem, documentary "Hackers" from 1995.

A "CISC to RISC decoder" isn't properly a decoder, it's a translator. What they had in the 80's were proper decoders: instruction's in, enable and clock lines out.

I don't want to argue about that. My point is just that the size (transistor count) and performance differences were massive at the time, like 2-3x the performance at 1/4th or so of the size.

Thanks for the explanation!