That is already the case. Very few people read or participate in threads more than 24 hours old. You're already walking into an empty room and having a conversation with yourself; all this change does is lock the door.
[1] For example, not realizing that RC4 is now hopelessly broken, and asking for a couple of recent references.
I don't reread the thread, but I will pay attention to someone replying to my comments even after several days.
Also, (not unlike this comment) I'll find myself late to the party and add comments to posts because I believe my thoughts might be relevant.
I actually treat HN comments to be a ledger of sorts. Like other industry forums around the net, it is a collection of some very strong minds on tech-related subjects, and a decent resource to check for opinion or tangential information on all sorts of topics. If there is a discussion on something that I have unique insight to, I will post on long dead threads just to know it was posted.
I'm probably an outlier, and this use case is probably not all that prevalent, but pending comments being purged without endorsements would make this forum unarguably more temporal, for better or worse.
(well I have an inkling and it's spelled pretty clearly in the OP but I fear it may be considered "gratuitous nastiness", to speak my heart)