Until you take a USB C cable with you in your laptop bag to connect with your portable USB C external monitor that gets power and video from one USB C cable and find out you have the wrong USB C cable with you.
Hate to say it, but it's entirely PEBCAK to not select a Thunderbolt cable when they want DisplayPort alt-mode. It's why the damn cable and marketing even exists in the first place.
99% of people only care about power (and optionally data), so that's what the spec focuses on.
Technically you could pipe compressed video over USB 2.0. It's not a part of the base specification though, so I don't see why either of these features are worth bringing up. iOS didn't even support class-compliant USB hardware until last year, so... it's interesting that you'd drill down on that.
Regardless, the fact that DP-Alt-Mode exists is not a refutation of the base USB-C spec. You're either spec compliant or you're not; same as Lightning without the license fee.
99% of people only care about power (and optionally data), so that's what the spec focuses on.