I am wondering how much the spec changes per M-chip iteration? I think that after some iterations, the spec stays the same, no?

Your intuition here is wrong.

Processors, not just Apple’s, change constantly between generations. The core instruction set doesn’t change, but they get new capabilities, not just with new instruction sets.

Remember the SoC isn’t just the CPU. It’s display engines, GPUs, storage controllers, video decoders and more.

Even outside of Apple, you’ll see hardware vendors provide Linux patches for their latest CPUs to keep working well. There’s some baseline functionality that works out of the box but it’s never a free ride.