The percentage of interesting articles has declined over time. Is that because I have overdosed a bit on the types of articles and discussions that appear here? Or simply because I have consumed more internet based news now than I had when I started reading HN? Who is to say.
Yes, sites in general can and have declined. However, due to the fact that I have not seen trolls dominate HN, and in my view the quality of HN is approximately as good as it has always been, I find it difficult to measure the quality of the site to a precision that would allow me to track it over time. In my own head.
The short answer is, this site is still among the best discussion news sites on the internet.