It's intentional, the 5 weeks old posts are not meant to be debated.

I think this is fine, I prefer to let a topic rest. Yes, I could go through all my comments from weeks ago and see if anyone replied, but that’s a little crazy. I’ll see you on the next topic if the same debate surfaces again.

Usually HN recycles a few debates for this reason and it pops up a few times over the course of months until everyone finally hashes it out. The breather is good, go to sleep, sleep on it, come back in a few weeks, and we’ll go at it again.

If I got constant alerts about comments, it’s going to devolve into an endless back and forth where no one took a break. Similarly, it also allows people to throw out their perspective against different people versus myopically trying to reconcile differences with just one person.

I’ve been downvoted to hell on a topic one day, and then several weeks later got modest amount of upvotes for the same opinion.

If you are active enough, then the same issue applies to 3 days.

Yes, and it's even more difficult if you started some thread that's got long, it's hard to find replies to your new comments that are there downstream. HN is probably one of few places where constantly hit Ctrl-F, so this certainly needs some improvement.