Maybe we need a "move to open discussion thread" voting button in addition to the karma buttons. Above a certain threshold off-topic but useful discussions could thereby be moved and orphaned from an original <-4 parent, perhaps leaving a link to the new thread or more sensibly someone else could just link to the new thread in another comment if there's any lingering topicalness. This would further discourage trolls because there would no longer be any correlation between their downvoted comment and a good discussion.
I just found out that a new website feature was implemented 2 days ago. If a comment is voted to -4 or below, it and all replies and downstream comments from it will be hidden from Recent Comments, and further replies in that subthread will incur 5 karma points penalty. The hiding, but not karma penalty, applies retroactively to comments in that subthread posted before the -4 vote.
This seems to be worth a discussion post since most people are probably still voting things to below -3 without knowing the new consequences of doing so.