Something like this would indeed be very nice. I would love it. I don't know why they call it HCY though. This is actually how HSL works (Wikipedia*) – every (light) color eventually turns into white (or the other way, into black).
When I was a student I wrote a document about this whole issue. Below is a demonstration how HSL and HSV (aka HSB) sliders work.
HSL sliders let you change hue, value and chroma independently.
HSV (aka HSB) always changes value AND chroma together if you use the so called "saturation" or "brightness" sliders. (And this drives me nuts.)
The picture below hopefully demonstrates what I just said.
Sorry for the German caption – here is a translation:
tatsächlicher Verlauf = actual gradation
Helligkeit = value (or lightness/brightness)
Sättigung = chroma (or saturation)
Actually, I had an idea for a whole "3D"-Gamut HSL color slider but I don't think this is relevant here. But in case anybody (programmer/coder) would be interested in turning that concept into a real color picker that will dominate every color picker that the world has seen so far, then let me know.