The mandate of the site is to be gratify intellectual curiosity (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...). Any website achieving such a thing will naturally grow.
> One important thing to realize is that we're trying to optimize HN for just one thing, which is curiosity
I’ve found it easier to practice creativity, for example, if I impose some boundaries.
We want the community to grow—it would be concerning if it didn't. But the engagement tactics you mention aren't necessary for the kind of growth we want. In fact we consciously avoid them. Certain kinds of engagement—probably just the sort that engagement tactics would juice—would harm what we're trying to optimize HN for (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...).
The interesting aspect of this, I think, is that HN doesn't need to pursue growth at all costs, the way that a startup would. It's not a startup, nor a business per se, but it's not noncommercial either—it's funded by a business that understands that it's more valuable with HN than it would be without it, and is smart enough not to try to squeeze profit out of it beyond that. YC's business interests are in having HN be as good as possible, not as big as possible. That's odd, and oddly satisfying. It seems to be a historical accident that HN ended up in that sweet spot. https://hn.algolia.com/?dateRange=all&page=0&prefix=false&so...
It's natural for users to engage less and less over time. However, holding the community to a certain standard keeps users from outgrowing the submissions and discussions. No one is too old, too mature, or too "smart" for earnest discussion.