I’ve heard that criticism but think it’s largely unfounded, Phil. There are SW casts - casting off a skiff and walk wading the flats. The Belgian cast used for heavy flies (at least that’s how it’s taught by many). Roll Cast pickup – with a Popper - ok that’s not strictly SW but it’s not light line either. The primary reason, I believe, for the tackle being 5-7 weight is to standardise the distances. There are a lot of MCIs down in Florida who have been very instrumental in the development of the CICP. I agree lots of MCIs probably don’t know much about SW fishing, but the reverse applies too. Hell some MCIs don’t seem to know much about fishing full stop!
From my perspective when I began as an instructor all my students were reservoir (stillwater) anglers. And nowadays they are mostly SW anglers. Most river trout fishers don’t bother learning to cast effectively.
I agree most people try to “overpower” heavier tackle. But the same applies to light tackle as well!
I do think that the same fly casting mechanics applies to short rods, long rods, heavy tackle, light tackle.
Cheers, Paul