The incredibly strict anti-trolling policies around HN do make it a great place to come back to years later. Though HN also takes care to engineer the social interactions here too. (scores are hidden, upvotes only until you get to a certain karma level)
I do tend to worry a little about bias and inconsistent enforcement of their policies, but also believe that we'd have to see that play out over time.
Without question the stated goal of "diversity" is noble, valuable and, deserves to be pursued. The problems arise in their definition of diversity. Does it mean what I think it means?
What about their definition of "safe"? We're really going to have to wait and see.
I think calling this the demise of reddit is a bit premature.
Though I also struggle to believe that deleting the extremes will make it a more interesting or compelling place to be around.
There's a reason that maniacs stop taking lithium.