I've seen this comment ever since I started reading Hacker News some 10 years ago. I've noticed myself getting warmed up to the argument over time, so I believe it's more an artifact of a user getting older than any real objective change in the site itself.
The necessary issue with gradual decline is that by definition there is never a point where you can definitively say it declined from one moment to the next. If you could, it wouldn't be a gradual decline, it would be a sudden decline.
Secondarily to this, if everywhere is in decline, there's no real way to tell as everything is relatively the same. In this context, HN is still ahead of reddit in terms of quality and content; reddit is still ahead of Facebook. Facebook and reddit however are very different beasts than they were a decade ago, let alone 15 years ago. Has HN had a similar decline as everywhere else? I don't really have a strong opinion on that, but it's a hard point to prove either way.
Personally, as someone that's been lurking HN since about 2010, the only differences that I can see is that HN seems less about startups than it was a decade ago and the comments are shorter and with less implicit good faith. The latter two though are happening everywhere, so I don't think HN is unique in that regard.
As an intellectual exercise, I went to a random date from the front page from 2009 and found someone complaining about Google's tracking and the degeneration of its search results:
You're right, which is the worst part of it; realizing you are part of the problem. I think the karma system is a big driver of it, you quickly learn what you "should" or "shouldn't" be posting.
An AI comment bot trained on past HN comments that can be unleashed on the actual HN to whore karma sounds like a great meta-humorous project - it would prove that as far as upvotes go, we have achieved the singularity.
If at the top of the list I see "Your appliance may be spying on you", it is not the platform that is a parody of itself - the farce is in this dragging world.
