They could have easily changed volume on their display - I had the touchbar but never liked the fact that you had to look away from your screen just to change the volume.

Changing volume is definitely one of those use cases where the physical function keys were much better IMO - It's one physical button press rather than touch the button, look down at the slider, and try to drag it into the right position.

I also use my laptop in a dock about 50% of the time, so end up just using the on-screen volume adjustment and keyboard shortcuts anyway... If Apple really believed in the concept they would have put the touchbar into their keyboards too.