> MP3 was synonymous with digital music in most of our minds for so long
I think this is probably the main reason for mp3's stickiness.
> second, even if modern equipment handles all the better codecs, a lot of us still have memories of times it didn't in the past.
Sure, and your dvd players didn't always handle x264. Things change :). It's been probably 10 years since new audio hardware had trouble with non-mp3 media.
> Third, I think some of the tooling around metadata is not as developed or ubiquitous for, say, an ogg container. There are ogg comments, but ID3 is better supported.
Granted, I think ogg will have worse support for equipment. However, I'd expect that aac in m4a will end up with the same level of support as mp3s do today simply because it's a lot more common than opus (and older).
> MP3 is no great sin.
It's not, I just don't like seeing generally superior tech getting sidelined because the inferior tech is more familiar. Perhaps that's a sin on my part :). I admit it probably doesn't ultimately matter if your music is 1MB vs 2MB.