I've run a social network for writers called Scribophile for almost 5 years now. People have said exactly what you're saying in this post about my site, all the way from day 1 to the present day. Too much negativity, people are out to get me, everybody is mean, person X is poisonous but I'm actually very nice, things used to be better in the "good old days".
But as an impartial observer (I, strangely enough, don't really participate in my own site), I can say that things haven't really changed that much in 5 years. There were nice people then, there are nice people now. There were jerks then, there were jerks now.
Any place you get messy, emotional people together in an anonymous social situation, this is going to happen. What is also guaranteed to happen--and I think your post is illustrative of this--is long-timers will grow bored and decide to drift off. A guy (I can't remember his name right now) wrote an insightful post about this and called it "evaporative cooling". It happens to every single community, in mine, and in HN.
Does that mean HN is on an irreversible slide into decadence? Not really. It just means you're over the community, and it's time for you to move on. In reality I doubt the general vibe of HN has changed that much over the years. Otherwise nobody would be here.
However as an administrator of a community, I can tell you that I certainly do not appreciate disappointed, emotional, "it's not the good old days"-style, long-winded farewells. If people aren't finding a site useful, then just leave, or send a private note to the administrator with your thoughts. Absolutely nothing good comes out of a public complaint-fest. Sorry if that sounds harsh and brutal.
This is why the community needs to always bring in new users. I see some people talking in other comments on this page about how to stop the new users, but if you aren't bringing in anyone new you are only shrinking.
In HN brings in no new users, it will be dead in 5 years.