A. Threads gets cluttered with nonsense posts, and irate responses.
B. Clutter from valid posts (near their origin), which become less relevant over time.
To solve these issues, would it be helpful to employ a default time expiration AND rating system on all posts? Perhaps a like/dislike button on each comment. This could enable high rated threads to have a longer TTL, and shorter for those with negative feedback. Over time, each thread would prune itself of unwanted comments or those which are no longer receiving attention due to becoming outdated.