Serious question: what is wrong with HN/Old Reddit style ? In my experience, that system manned by competent moderators delivers satisfactory discussion.
I also have this question. Moreover, the presumption with "complex" discussions is that the participants will seek to engage despite (relatively) small barriers, which presents (imo) an existential challenge to this solution.
Indeed, threaded demands more effort from the user, hence why I usually push Discourse for casual communities - flat works better there. But here we are talking about "complex discussions", for which flat is frustrating. Market segmentation !