You have a point with HDMI because that's a digital cable.
Ethernet is also a digital cable.
Cable quality when it comes to digital doesn't matter, but with analog it is far more sensitive to the quality of the materials used.
The walkmans are fine, I actually have an NW-A306 ($350). To be 100% clear, the other walkmans are PRETTY MUCH the same, you are mostly paying for a better build than anything. It's a quality device but it isn't like you really need anything better.
Your money is better spent on speakers or headphones over anything else. You can have the fastest car in the world but it doesn't mean anything if you don't have tires that can handle it.