>Mathematically Y'CbCr is a lossless conversion of RGB.

Assuming infinite precision, sure.

Interestingly however, there's a closely related color space called YCoCg, that actually does achieve lossless coversion to/from RGB (this lossless variant is often referred to as YCoCg-R, the R stands for "reversible"): https://en.wikipedia.org/wiki/YCoCg#The_lifting-based_YCoCg-...