The problem is that no one, not even those of us who spend excessive time here, sees every thread or even every major thread. HN's front page is a tiny aperture given the firehose that flows through it. We added 'past' to the top bar a few years ago to give people a way to catch up on the biggest threads they missed. Unfortunately relatively few people use it. For example, this month about 4% of logged-in users who've viewed the front page have also used 'past'.