HN is an experiment. As a rule, a community site that becomes popular will decline in quality. Our hypothesis is that this is not inevitable—that by making a conscious effort to resist decline, we can keep it from happening. - https://news.ycombinator.com/newswelcome.html
pg wrote that over 15 years ago. I've been saying for (a mere) 10 years that we're trying to stave off the arrow of internet entropy: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que... – not doable forever, no doubt, but doable for a while if we expend enough energy.
Based on experience so far, the way to do this is through a combination of a dedicated community, software that does what software can, and moderation to bump the system out of its failure modes.
Trying to hide from the outside world is mostly not the way. pg used to do tricks like Erlang Day on occasion, but having that as the main strategy would be like trying to avoid infection by never going outside. Far better is to have a robust immune system, if possible. Trying to avoid (resistible) pathogens weakens the immune system, and hiding HN from new users is a path to decrepitude. The latter is probably a greater threat IMO, and too easy for established users to discount.
Spam, and its cousins like content marketing, could kill HN if it became orders of magnitude greater—but from my perspective, it isn't the hardest problem on HN. That's because of the dedicated community, which flags these things, reports them when they escape in the wild, and is vigilant about quality. Without such a community, HN would have died long ago.
By far the harder problem, from my perspective, is low-quality comments, and I don't mean by bad actors—the community is pretty good about flagging and reporting those; I mean lame and/or mean comments by otherwise good users who don't intend to and don't realize they're doing that. There's an unholy dynamic between those and the upvote system, so worse comments often get upvoted more than better comments do—often enough to choke the threads with weeds. That's the high-order bit and what I spend more time worrying about—not Cassandra E Oakley and her trading system*, nor the latest startup voting ring and whatnot. If those ever become the high bit, we might be doomed, but we should see it happening long enough in advance to readjust.
p.s. (@dang doesn't work - I only saw this by accident. Well, not by accident because it ended up at the top of the thread)
* https://news.ycombinator.com/item?id=39425371 - unkilled for the occasion