This thread disagrees with you. HN is slowly becoming like Reddit. Down-vote all you like, it doesn't change the facts.
Given that Reddit is, at this point, nothing but a thin, MOSTLY-public-friendly landing page, papering over the world's largest porn portal, it's unsurprising that people who value original thought have left and found other places to discuss things of interest. And when THOSE people leave, the people who like to troll THEM have to find other places to do it, too. Slashdot has also suffered from this diaspora.
What, so that thousands of HN readers in search of an authentic open web experience can trample them into dust?
Tagfam.org might qualify, though it's a niche subject. Oldest education related forum around, afaik. Though it's a set of email lists, not boards.