> In a sense our eyes experience contrast on a sliding scale

There's a whole visual center to check the amount of incoming light and adjust your pupils for you. It's intentionally reactive.

> and there is no absolute zero in the system.

There maybe is. I think we call that "blind."

> broadcast systems historically used 16-235 as their luminance range for SDR

Mostly because it was a fully analog system and these all translate down to signal voltage. Jokingly NTSC used to be referred to as "Never Twice the Same Color" due to being a compromise bolted onto the side of an already compromised system.