"I decided that analysis should focus on the higher, more conventional rates – 48k and 44k1" - opus is always 48khz, so that doesn't mean much.
1. Poor sensitivity in bass and treble. See:
https://en.wikipedia.org/wiki/Equal-loudness_contour
2. Limited ability to hear multiple sounds simultaneously, or almost simultaneously. See:
https://en.wikipedia.org/wiki/Auditory_masking
Bernhard Seeber has some videos on Youtube with demonstrations of auditory masking:
https://www.youtube.com/watch?v=R9UZnMsm9o8
https://www.youtube.com/watch?v=bU0_Kaj7cPk
The only fair way to evaluate lossy codecs is with double blind listening tests.
There is such a broad spectrum in thr word mp3 that you need to be more specific. I can absolutely pick the difference between certian high bitrate mp3 encodings and wav files, however a different mp3 with exactly the same bitrate, effectively indistinguishable.
Bad mp3 encoding is a problem, not one I have experienced recently though. I think the bigger issue is people will rip a music video from youtube, then instead of extracting the existing audio stream into it's own container will reencode it. Mp3->mp3 encoding will be lossy just like any other encoder.
And who are therefore forced to hear terrible audio because the compression method only considers the majority.