Yes but it's not clear how to get it right.
Maybe also disable replies to old comments and make them a different color?
To be fair, we could never figure out how to get this right on reddit either, but HN has a better comment sorting algorithm than reddit so it might actually work here.
If they wanted a notification system I'm sure they could have implemented it.
On the other hand, it can be hard to discover replies, and certainly new/interesting comments in large threads, when you want them. I'm currently working on a new-comment-highlighter (draws a colored bar to the left of comments since you last viewed the page) as one step toward helping with this. If anyone wants to help test this, email hn@ycombinator.com and I'll switch it on for you.
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.
Or just have a bot account post a link to the previous discussions. Even that would work, just so that dang doesn't have to do it manually.
honestly, i just feel bad for you. what appears to me to be a repetitive google + copy + paste on your part is anathema to me, and i was trying to help stamp out such work.
i think this has value, but im worried the amount of work involved grows (at a minimum) greater-than-linearly as HN grows.