Because there is probably a lot of overlap between those who think Youtube comments are terrible and those who post terrible Youtube comments.

People might slag it in public, but I think a lot of people secretly enjoy leaving abusive and trollish anonymous comments. It's not like all those comments were the work of a few bad apples. The sheer volume would suggest otherwise.

You're making a very basic logical error. Youtube comments on the whole may be horrible, but that doesn't discount the possibility (in fact, I'd say certainty) that some youtube comments could be good, even valuable. There are many channels on youtube where the comments are fairly high quality, even by HN standards.

The downside is that the people who put in the effort to nurture high quality comments on their channels, and the people who have been high quality commenters on youtube are the most negatively impacted by these changes. And the negative impact on the highest quality of comments might end up reducing the average quality much more than any moderate impact on the volume of the lower quality comments.