I'd bet the closure of other walled gardens like reddit drives traffic more than anything of content.
These comment sections follow the same trajectory of all comment sections since the birth of the internet.
While HN is part of the set, I think we need to look larger. Is it just HN, or is it everywhere? If it's not everywhere, what percentage of sites is it? What are the sites that don't have this issue like or do different.
I think part of it is that HN has grown a lot because of a lot of other sites getting worse. The problem is that growing makes it worse. Then add on top of that there are a lot of high value targets here on HN that bots try to target, financially or politically.
Internet is a messy place now. I remember years and years ago, like early 2000s people thought that things like astro-turfing weren't real, and governments were not targeting individuals/groups on forums. How nieve their were.
While it has always been a thing on here to some degree, the one growing trend I will note is that people are increasingly more forceful in demanding that everyone know about the things they aren't interested in, and more demanding that other people don't talk about things that don't interest them.
It's like people can't just skip a submission on the front page. They absolutely must go in to every discussion to tell everyone how much they don't want to read about or talk about that subject, and how much they want everyone to stop talking about it.
It's tilting at windmills. Right now this can mostly be seen in AI discussions where just about every AI-related submission will have a number of people declaring that it's all so boring and they wish there was an HN ban on such submissions. Cue someone foolishly believing them making yet another "HN without the AI" variant that sits unloved.
But the same thing could previously be seen against things that people don't want to be a thing. When WWDC happens people will announce that they're sick of all of the Apple news. The same for Google during I/O. Etc. People are against some thing and they really, truly think they can shame or scold others into falling in line.
And then LLM happened. And HN got more LLM writing, more LLM articles, more complaints about LLM-written material, plus AI became a dominant subject of conversation. LLM is in addition to the other negative changes.