I think something you miss about the HN dynamic is that posts that fall off the front page quickly almost invariably have low-quality discussion compared to the ones that stay on the front page for longer.
Some of that is because low-quality discussions fall of the front page quickly, but most of it is because the first comments on a post are always low quality, because they're disproportionately from drive-by posters who read the title but not the article or the existing comments. If a post falls off the front page quickly, then those comments become the only ones that get seen, which means even if someone comes along with a more thoughtful comment (unlikely) it ends up buried with no upvotes and no replies.
If you let a "dupe" that's finally catching on stay on the front page for a while, the conversation invariably becomes better as the thoughtful commenters arrive and begin to leave their mark on the conversation.
In other words: just because 5 threads are ongoing and uninteresting doesn't mean that this one didn't stand a good chance of having a good, useful conversation if left alone.