HN threads are closed to new comments after 2 weeks, mostly as a way of preserving history. A forum discussion is most meaningful as a snapshot in time.

It would be weird if people could go back later to ancient threads and add comments based on information that wasn't available at the time. Actually, you can go back and look at very early threads, before this restriction was put in place, and it was already weird. Imagine what the original Dropbox Show HN would look like by now if we didn't restrict this.

All that said, I think you guys are on to something interesting with thoughts about how the software could provide better access to the history, beyond just posting links to past discussions.