It depends on the size of the community in my experience. Old school bulletin board style fitting are great once trolls and manic megaposters are kept under control but if they've too few users they tend to die and if they've too many they become impossible to follow
You mean insane; every sane discussion place looks more like subreddit (or, often enough, is a subreddit).
> 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).
> They could, but rareley do. On Reddit this requires clicking many links.
It doesn't, unless you're using the new UI, at which point it's kind of your fault anyway, because Reddit is entirely unusable as a discussion place with its new UI. You have to click to load more than 3 anything, and half the time it refreshes and loses your position anyway.