Lots of pseudonymous forums are reasonably civil. I suspect the problem with Youtube's comments is more an absence of signal than the presence of noise. Videos are far more accessible to the barely literate, "People who watch videos on YouTube" has no community identity, and there's no external purpose to the commenting to overcome the weakness of text as a medium to discuss cat videos.
I'm not sure why Youtube even bothered allowing text comments in the first place, let alone why they didn't go to video-response only years ago.
There also seems to be an impermanence to commenting on a video, especially one that's popular, because responses (unless sufficiently up-voted) don't stick around, and there's no way to easily search through them.
So a YT comment stream takes on a party line vibe and (IMO) there's not much point taking the time to write something thoughtful.
Mainly when I have commented is on videos on niche subjects when I'm reasonably sure the video poster/artist/producer will see it.
There was a period when I was able to Have A Conversation in YouTube comments but I was never able to do it more than a few times. On the other hand, I see people having legitimate conversations on random occasions quite frequently.
I'm not sure why Youtube even bothered allowing text comments in the first place, let alone why they didn't go to video-response only years ago.