I am not sure whether it has been improved, but back when the Ibiquity HD was being deployed, the codec actually synthesized sound above something like 5 to 8Khz at the receiver, all depending on the bitrate choices and band, AM having a lower overall available bitrate to work with.
Someone noted a song by the Bangles, which featured a little finger cymbal. (Two discs worn on a finger and thumb)
That sound was missing and or rendered poorly. Turns out it offers very little sound under 5Khz. The codec parameters were increased to address those few cases, but are still below 10Khz, I believe.
The other discussion was quality over choice. Fact is a full FM bitstream performed great. More than good enough to really enjoy in a car. Most people would trade the losses for lack of fuzz and noise present on analog RF.
"CD Quality" was, and today is not accurate at all in the technical sense, but the overall feel is more like a CD than it is analog media, so... that is what they went with.
However, being able to offer several streams meant multiple branding, commercial free streams to promote digital radio, and, and, and.
Mostly, that discussion resolved to choice being king over quality. Improving quality at the expense of choice = wasted bits. That is by far the dominant view in broadcast.
A little thought experiment explains why:
Say you have a radio, and you get two stations. One offers compelling content, but the quality is low. The other is amazing good quality, but is just not compelling.
Which do you listen to?
Given others find the same material compelling that you do, which do you believe they will listen to?
And that is why peak quality on broadcast is no longer a thing.
Streaming services can offer compelling and quality choices to a higher degree than broadcast can, which means generally higher quality, even when available bitrates are, or could be. comparable. But, if they are ever pinned, they will favor choice over quality in a pinch, because compelling content wins over quality nearly all the time.
I pretty much gave up on radio around this time too. Being able to carry a good music library around meant just using a car system as an output device.
For a time, I did enjoy live or daily broadcasts, talk, news, sports. But, those carry such a high commercial load, and when phones and data plans began to favor streaming, I switched completely.
...all of which is why I am not too current on how HD Radio performs today. Next time I rent a car, I will have to scan the dial to see what, if anything, has changed.