> Trees devolve into a 1on1 discussion, also other branches can have the same discussion, leading to redundant arguments.

1 on 1 is a feature, as it's never truly 1 on 1 - anyone can jump in instead of just listening to the conversation.

As for other branches having the same discussion, the solution is the opposite of what you're proposing: instead of flattening tree structure into a line, embrace the nature of any discussion (or knowledge accumulation effort) and make it a directed graph. I'm yet to see it done in production (outside 4chan maybe), but that's the right way.

Or, you can just ignore the problem and let people having their redundant discussion. It's better than what all those flat forums you mention tend to do: closing topics with aggressive admonition to use a search feature (which is near-universally broken anyway).