The ISA is the contract or boundary between software and hardware. While there is a hardware cost to decode instructions, the question is how much?

As all the fanbois in the thread have have pointed out, Apple's M series is fast and efficient compared to x86 for desktop/server workloads. What no one seems to acknowledge is that Apple's A series is also fast and efficient compared to other ARM implementations in mobile workloads. Apple sees the need to maintain M and A series CPUs for different workloads, which indicates there's a benefit to both.

This tells me the ISA decode hardware isn't or isn't the only bottleneck.

Whatever the hardware cost is it must pale in comparison to trying to implement the instructions set directly. Wasn’t that the huge benefit of the Pentium Pro?

The main reason to have both A and M series chips, I think, is related to which custom processors are available as well as how many of each. There’s no point in putting the full GPU from an M4 into a cell phone, it’s just gonna waste a ton of power.