I'm pretty certain the purity of copper becomes inaudible to the human ear much faster than companies bilk idiots with too much money. we're talking hundreds of dollars for speaker wire.

Ehh just because it's on or off doesn't mean the cable has no effect. Thinner wires means more cable loss meaning it's easier for an on to look like an off. Additionally better cables can have better/thicker/more shielding making cross talk on the wires or interference from outside sources less impactful. Not to mention insulation protecting the cable from damage. Source: am EE for a company that does video transmission.

At high enough speeds, digital starts looking like sine waves. 😕 Course, HDMI isn't quite there yet unless the latest stuff is bumping up against it.

This is a problem people who are doing PCB layout for RAM have to worry about. The traces have to be nearly the same exact length else the timing is off due to trace impedance. If you look at a PCB around memory chips, you'll see the traces doing little serpentines to try can keep the trace lengths the same.