Seasonal variation is a possible explanation (see
https://en.wikipedia.org/wiki/Eternal_September ). But I'm not sure if that the explanation.
There is a nice comment in a thread of a few years ago, that has an ascii-art graph of the quality of the HN comments [1]. The values are made up, but it show that the quality oscillates, and people complain when it's descending. But it will bounce! (I hope.)
So, I'm not desperate, but a small rant of another user makes me not feel that I'm the only one that sees a problem.
[1] I can't find the original comment. Do someone have it in a bookmark?
[2] Lots of karma (or more precisely at least 1) for someone that can make a real graph using some kind of metric to guess the comment quality. (Length is a good proxy. Points need a correction due to the increase in users and karma inflation. Deep learning?) Does the real graph show seasonal variation?