First, I wouldn't call MP3 "low-quality". There are some limitations to MP3, and it has lower fidelity than other codecs at the same bit rate, but MP3s are crystal clear when done right.
AC3 has higher fidelity at the same bit rate, and can be used at higher bit rates, but the difference between AC3 and MP3 is not exactly night and day. AC3 is roughly similar to MP3 in terms of quality and bitrate, but that's not a bad thing.
As mentioned elsewhere in the thread, the audio format in cinema wasn't technically AC3 but something quite similar.
MP3 has fatal flaws on some audio and can't compress it transparently at any bitrate - notably happens with cymbals. Some of this is because there's a maximum bitrate above which it's technically out of spec.
There is a reason we don't use it anymore. AAC/Opus don't have these problems.
Valid reasons... but definitely not "low-quality".
When we're talking about "transparency" here, we're talking about finding small percentage of samples that can be ABX'd by people who are specifically listening for artifacts. For plenty of source material, you get transparency with MP3.
Yes, for almost all source material this is true. Unfortunately there's definitely killer samples out there that can be distinguished at 320kbit latest LAME with no effort at all.
Also, rather than increasing complexity I think the newer better codecs are actually simpler than MP3, since we learned which parts of it helped and which didn't.
AC3 has higher fidelity at the same bit rate, and can be used at higher bit rates, but the difference between AC3 and MP3 is not exactly night and day. AC3 is roughly similar to MP3 in terms of quality and bitrate, but that's not a bad thing.
As mentioned elsewhere in the thread, the audio format in cinema wasn't technically AC3 but something quite similar.