I can distinctly remember several times the comments on an article were written by absolute experts in the material, full of insight and interesting content.
I could believe comments like those have become rarer.
Reading your second sentence strictly, we can infer that to you, quality means (a) expert-level familiarity with a linked article, (b) insightful -- difficult to generalize because insight is different for different people, or (c) full of interesting content.
Of the three of those points, only one of them isn't dependent on the perspective, knowledge level, or interests of the reader. What is insightful to me might be common knowledge to you. What is insightful to you might be tediously stupid to me. What is interesting to you might be boring to me. On, and on, and on.
I'm just expressing one vote that I agree with some peoples' consensus, that a particular type of quality of HN has decreased; and it's a type of quality that I value.