I've "felt like hn is declining" many times in the last fifteen years. Notably, when I find a whole page of slanted, snarky bad-faith posts of various sorts.

The thing someone pointed out, however, is you get particular bad threads through the process of them being seeded that way rather than things always being bad. I think that's why hn's moderation is often thread-by-thread rather than comment by comment.

And I can find some threads here with quality comments still. So quality is a difficult thing to measure.