DisplayPort "fast AUX" stream is more than capable of returning audio or even video data to the "source" and some videoconferencing rigs even implement this. But I don't see why the industry would be motivated to adopt and consolidate around that standard when HDMI's bidirectional features already exist and are widely supported.
Aux is its own channel. And one I seriously wish for better used! It seems to only be USB2 capable but even that is rarely implemented!! So sad.
But one of the neat things about DisplayPort is that it's a a packetized protocol, that many streams can go over. Which is why there can be multiple daisy chained monitors!
So hypothetically the computer could stream out all manners of high bandwidth data. It's been neat to see a USB4 adaption, where DisplayPort tunnels USB4 for data coming from the computer, and the aux channel is used as the 480mbit return channel.
I don't really have any knowledge of why the aux channel is regarded as not capable of multi gbit/s, and maybe that's not even true.
Given what a terrible inflexible trash porting of the past HDMI is though, sure makes me wish folks would do more with all the possibilities DisplayPort has. And it makes me really really wish consumer electronics would please please please offer some DisplayPort. Given that usb-c mandates DisplayPort as a required alternate mode video system, maybe possibly hopefully this will be some leverage that starts prying these HDMI rent seeker's stranglehold open.
The physical layers of everything are converging. The breakthrough we need is on the logical layer, with peers describing their capabilities and working out a sensible, predictable suite of resulting functions. Right now there are _zero_ conventions for what the DP Aux stream should do, so random member of the public has no expectations. HDMI for the most part does what normal people expect. USB has maybe a few too many possible outcomes ... for example yesterday I discovered that my Pixel phone was charging my MacBook, instead of the other way around.
Unfortunately working out a set of norms and expectations that are useful to users is harder than just making bits go over wires.
