I’m not sure if e.g. Displayport even has the capacity for link training (and there are USB-C to Displayport cables that have to support legacy devices that know nothing about USB); HDMI (until 2.2 or so) definitely does not.
It’s ok to not agree with the USB-IF’s tradeoffs in their solutions, but denying the complexity of the problem space can be a hint that you don’t sufficiently understand it to pass that kind of judgement.
Intel has a flow for how link training is done on DisplayPort.
Probably shouldn't be surprised but it involves communicating over the AUX channels. Is this something that a sizable % of computers can do? For some reason I thought aux channel was semi free for use, that it could be for Ethernet or USB in a pretty naked form. Didn't realize that needed mode switching?
Ah, so maybe DisplayPort has mandatory link training then, which would indeed allow unmarked cables.
But to GPs point, there still needs to be a way to tell the source that a given cable is a USB-C-to-DisplayPort one in the first place. So why not include the metadata on what signal grades it’s rated for in that same indicator? That’s exactly what e-markers are.
This means cable might start at speed which is too high and get horrible errors if the cable is bent.
And even with Ethernet, that autorating has plenty of downsides. I've had a cable which I accidentally nailed through, so sometimes it could only connect at 10Mbps. It took me a long time before I realized it needs to be replaced, precisely because it'd just downshift instead of throwing any errors.
By attempting to link up at the highest supported speed and downshifting if there's no valid signal? Ethernet had this figured out decades ago.