120 lines (max) will be sampled (roughly) as 120 samples 3 pixels wide, that's it... if not, we'll get 360 effective chroma resolution even from VHS, that had (when lucky) around 30...
Also, luma sampled at 6.75MHz gives 720 pixel, that are not the effective luma resolution... I'll kill to have a VHS tape with an effective horizontal resolution of 720 pixel (luma) and 360 (chroma)... don't you? (^^,)