You are confusing digitally obtained data as lossless. 0 = 1, so to revert 1 = 0. To obtain this data you must first sample, and then sub-sample. There are many analog sampling stages before you even begin to see data.
Digitization is lossy by definition.
You are confusing subsampling with sampling. Every digital signal is sampled (and thus carries some error compared to some analog source). That is irrelevant to this discussion imho.
The act of converting (digital) R'G'B' to (digital) Y'CbCr is not digitization. No ADC is involved. No subsampling is involved unless you choose to (spatially) subsample the chroma channels to e.g. 4:2:2 or 4:2:0.
There is (usually) some loss in converting R'G'B' to Y'CbCr 4:4:4, but this loss is not what an engineer would call "subsampling", it is quantization and clipping. I routinely do such conversions in MATLAB that is lossless (due to using floating-point numbers).
For further background, I recommend Charles Poyntons book "Digital video and HDTV: Algorithms and interfaces", p.p 90, "Chroma subsampling".