What is this "Quality Loss" they speak of regarding downsampling from 48KHz to 44.1KHz? When resampling is done properly, you'd only lose very high frequency c

What is this "Quality Loss" they speak of regarding downsampling from 48KHz to 44.1KHz? When resampling is done properly, you'd only lose very high frequency content (frequencies above 22050Hz). Human hearing range is usually documented as up to 20KHz, but some young people can slightly hear higher frequencies.