Chroma subsampling reduces the density of available luminance approximations
Chroma means color, and color subsampling is used to avoid taking information out of luminance channels because they are more important, so it is actually the opposite of what you are saying here.
There simply aren't enough bits of precision in the luma encoding for good gradient support most of the time, chroma fills the gaps, and chroma subsampling produces artifacts.
I think you're conflating a few different things. Chroma doesn't fill gaps, low resolution chroma channels introduce artifacts of their own.
This is spatial resolution, 10 bit color channels is quantization resolution of the values. Everything contributes to banding artifacts, which are just noticeable changes in values when that are meant to be perceptually smooth, but the luminance channel is the most important, which is why it isn't subsampled.
These are fundamentals of image and video compression and not unique to webp.
Chroma means color, and color subsampling is used to avoid taking information out of luminance channels because they are more important, so it is actually the opposite of what you are saying here.