I look at it this way: it's designed for engaged, prolonged discussion if and only if people continue to be interested enough in it to remember. If they aren't, activation energy falls below a certain threshold and attention naturally moves on to something else. That seems healthy for curious conversation, which fits the prime directive of HN (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...).
What we don't do is resort to technical tricks to keep that energy buzzed up. We're optimizing for curiosity, not engagement.
Which is precisely why this is my favorite message board on the Internet. I love that I never have to collapse comments in HN threads because they're witty or off-topic.