USENET with a threaded newsreader like "trn" provided the optimal experience here.

You saw things in their threaded context, but it remembered what you've read and there is a direct action to "go to next unread" that will jump around and follow the fringe. You don't have to open individual root posts.

It wouldn't work so well if you expect to read sparsely though. People used moderation and killfiles to prune out garbage. The death of USENET was in many ways the flood of posts that made this no longer feasible.

The other missing thing here is topics, i.e. newsgroups. HN is not as broad as USENET as a whole, but also not as narrow as one newsgroup. These groups are what you would open, then skim through all the messages in that forest, catching up on what is new since last visit. HN topics are too narrow to want to bother reopening each one to catch up, but there is no collective layer above them to help find your own sparse subset of worthwhile HN conversations.