The mechanism would be whatever cognitive bias makes it seem like things are always getting worse. I think nostalgia plays into this, and also status—everyone enjoys feeling superior to late-comers. It's one status you can never lose!

I think the fact that people have literally been making the same complaint over and over about how HN is turning into Reddit since literally before it was even called HN is strong evidence (not proof, but strong evidence) that that perception is bogus. Had it ever been true, HN would have become dramatically different a long time ago. Not only the perception but the phrases people use to express it have been unchanged for almost 15 years. That's an internet eternity.