One of the biggest reasons I can think of for the eventual decline of HN are the result of these words;
>>>On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.The problem over here is that people's version of what gratifies their intellectual curiosity varies. What I find interesting is deeply subjective and I need to acknowledge that to contribute anything, anywhere. The same thing goes for HN in general.
HN is starting to attract people outside of its core audience and in doing so it inevitably leads to the point where people come in whose mean of what's interesting is different from the mean of what's interesting for the core audience.
It isn't their fault, but this process feeds into itself. So, if I see a post here on HN on death. I have this tendency to post up stuff related to death. Regardless of the fact whether or not they adhere to the spirit of the guidelines. People see that and they are more inclined to upvote it, since there is already content like that on the front page. So, it accelerates.
You might have noticed that the front page of HN always follows a pattern there are a series of similar posts which come up and stay there over the course of time. This is a good thing and a bad thing as well. The trick is in stopping it at the right time. That's where the community and the moderators come in, but if the majority of the community don't know any better than that. This means that the community can't self adjust. The moderators then have to play a stricter role, which is inherently negative in the longer run.
At the same time, if you define those words too strictly then HN won't be HN, anymore. This is the underlying paradox that needs to be solved to avoid that decline.