Even if what you said is true, you aren't going to be losing any colorspace by converting YCbCr > RGB > YCbCr444. If image quality is lost, then it is because the conversion to RGB caused black crush, not because of the conversion back to YCbCr. Again, the final quality is A) based on your display and B) how your display is calibrated. You can calibrate your display so RGB looks great, even with black crush. On the flip side, you can also calibrate your display so YCbCr looks great, even with two conversions. Further yet, it still boils down to preference. You can't justify "better" or "worse" image quality, because it is a matter of perspective. You can't measure an individual person's preference.