A specific suggestion I have is to establish a community norm of downvoting those participating in hopeless conversations, even if their contributions are high-quality

How do you define "hopeless", exactly ? Sometimes, a high-quality post in response to a troll's or idiot's thread can be quite helpful to other readers (and lurkers), who aren't trolls or idiots, but who are just misinformed or new to the topic. I personally have been such a lurker on several sites, but I do acknowledge that my personal experience is not statistically significant.