Yeah. More chat than forum, in the sense that people rarely venture to reply on what is older than a day (apart from Ask/Show HN; both hang a little longer)

I don't get your solution. It seems to me that HN threads are already tree-like.

Yes, but they are a bit too easy to split, which means that a good chunk of comments end up getting ignored.

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 ?