"HD" phone codecs seem higher quality than the example given.
That's true, but in this era of high quality voice and video calls, it's not that uncommon for someone to want to play a song or even a live instrument, so some capability for handling that intelligently seems important.
The mics built into modern smartphones aren't limited to the fidelity level of ancient telephones, so it seems reasonable to hope for more.
I believe the limitation was actually not the mics. You can fit a lot more phone calls into a given bandwidth if you heavily restrict the bandwidth of each phone call.