(Outside of the context of HN I don’t have anything to say here about the industry as a whole. Someday in another more appropriate post about the tech industry I will, though!)
I will just speak for myself, but a large part of that "uptick in negativity" is from professional insiders who changed their views, not necessarily from newbies "poisoning" the site. Tons of people who were incredibly optimistic about the role of tech and specifically the startup ecosystem in society 20 years ago feel very differently today.
At the time it was a massive conversation around whether reddit should have to host content its advertisers didn't agree with, except they had heavily contributed to killing off old forums whose free speech was not compromised by advertisers. This was part of a larger conversation about free speech protection online and right-to-platform which still ensues. Corporations frequently cite their own free speech interests while suppressing speech of individuals and organizations. The current organization of the web is just perverse and tenuous.
Hacker News is quite unique in that while it is an aggregator, I have never felt that staff moderation crossed over into censorship, even when I've been on the receiving side of it. The moment dang leaves and pg retires, it's over. Tom Howard might do OK for a while, but the reason HN, despite eternal September and growing amounts of negativity, spam and astroturfing, continues to be a fun place on the web is because Dan is the man and fully embodied Paul Graham's established foundational principles.
We all need to be thinking about what kind of internet we want our grandchildren and future society to inherit and start building today.
A lot of Reddit problems were own-goals. Lets have a civil war over being a free speech or being a brutally censored speech site, I'm sure the civilians trapped in the middle will love the experience. Spoiler -> They did NOT.
But humans can always be replaced by bots if you're selling advertising views, LOL. HN doesn't sell ads (or at least none have ever gotten past my ad blocker) so there's fewer bots here so the decline due to bots is slower here.