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.
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.