Forums and chats do have a solution to too big size : they can just split into others (or subforums) once the linear discussion becomes unmanageable.
I don't get your solution. It seems to me that HN threads are already tree-like.
I sometimes read the new ones by first reading all of them, and later manually scanning every comment newer than X hours, but it's tedious.
I guess a system that would automatically hid old comments (except their immediate parent) might work ?
I know there are a couple third-party services that you can sign up on to monitor that for you (they email you when someone replies to you, AFAIK), but I opted to just go with the flow and hit my "threads" link every now and then to see if someone has replied to something.
Agreed - built-in, opt-in HN notifs (or easy-to-opt-out) would be good to help with what OP is asking about, IMO.
Is there a middle ground? I don't know.
As for old threads getting bumped, I think this is here equivalent to reposting the link (or re-asking). The old thread is almost "dead", but this is how it works "in real life discussions". So maybe that is not so bad. This is (as a call it) a (fast) stream model, where there is no (direct) accumulation : it is more twitter than Stack Overflow, more blogs than Wikis.