The fact that you've been reading this comment forever does disprove it since hn has been growing steadily for the past 10 years.
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:
https://news.ycombinator.com/item?id=979033
So in that regard, nothing's really changed but the severity of the problems.