In addition to my sibling comment: HN also steps in to quash developing negative patterns in all sorts of ways. There's a long list of banned sites, there is the flamewar detector (though I've ... questions ... about that), dupes detection (or flagging), there are weightings and penalties given to various sites. I believe also some keyword and other patterns are looked for as well, "Reddit" being among ones dang's recently discussed.

So yes, occasionally some new pattern or trend will emerge, but HN adapts to those fairly quickly.