4:4:4 doesn’t mean twice the compressed bitrate, however. First, chroma residual already takes a minority of bits - luma residual and prediction info take way m

4:4:4 doesn’t mean twice the compressed bitrate, however. First, chroma residual already takes a minority of bits - luma residual and prediction info take way more bits and those are unchanged going to 4:4:4. Second, quantizing chroma more so there’s the same number of chroma bits as there would have been in 4:4:4 is approximately equivalent as subsampling chroma in the first place (okay, its actually slightly worse, so yeah you need maybe 1% greater bitrate to match quality with 4:2:0 with modern codecs on sequences for which 4:2:0 subsampling is transparent.)