Reading the other reply thread with slowerest gave me another possible solution, too. Perhaps the comments sections for journalistic pieces from organizations

Reading the other reply thread with slowerest gave me another possible solution, too. Perhaps the comments sections for journalistic pieces from organizations like Ars, NPR, NYT, local news, etc could be more of a competition (like Slashdot). Top 300 comments get preserved, leave it open for a month with no comment limit and some light moderation, and let the conversation go wild (I like Reddit's system for this), then delete all but the top 300 at the end. Adjust "300" and "top" to fit your organization's needs, just make sure they're clearly defined. Would also help limit the scope for an ML-based solution, too. :)