I think large scale makes it impossible to be good. Small scale doesn't guarantee good.

Reddit, slashdot, and a whole slew of others went through an HN-like phase of high-quality discussions and content before turning into cesspools.

I think what makes HN unique is benevolent sponsorship. There isn't pressure to grow. It's recruiting for Y-Combinator, which eliminates most of the normal pressures.

i also think it's a content thing. the content of HN doesn't lend itself to lunatics most of the time. i say "most" because it certainly happens (see the early covid threads and the hatred spewed for anyone questioning or even just wondering aloud about the lockdowns)