An important question for a retrospective posts is why has HN not degraded as quickly over time as other sites?

I think the key game mechanic increasing the longevity of Hacker News is the minimum karma threshold necessary to downvote comments. In effect, it requires new users to undergo a period of socialization to community norms. During this period they train in recognizing what the schema for comments containing negative informational value looks like. First out of fear, then out of mandate as an acquired power.

If you ever want to keep a community alive, implement similar withheld powers of negative punishment and adjust the level of input work required to earn them.