When threads are closed you have a lot less moderation work to do to keep the site from being infested with spammy outbound links
It's unfortunate but the value of the odd one or two people commenting on an old post isn't great enough to outweigh the need to easily keep the site spam free for many sites
In this context, the Guardian's "Community standards and participation guidelines" page is a good read.
Note that I'm not claiming that old comment sections should always stay open forever, just that great grandparent is not on its own a good argument for closing them.
So here is a second argument: maybe the site's moderation process relies on flagging by ordinary users, and maybe the modal ordinary user cannot be coaxed into using any method for finding out about new comments except for scanning individual comment sections.
I jumped into this conversation because I suspect that it is worthwhile to try to keep comment sections open as long as practical, so if I were in charge of a discussion site where the assumptions of the second argument were valid, I'd try to avoid the conclusion: for example, I might give users a way to subscribe or express an interest in getting a notification (by, e.g., email) whenever a new comment appears in a particular comment section; then I'd keep a section open for new comments as long as it has subscribers; and then I'd wait to see whether the subscribers flag enough of the new spam in the section to keep rate of spam in the section to an acceptable level.
But that's people. Spam bots scan sites for open comment forms, and having them on every old page increases the attack surface. Usually the only comments on old pages are tons of spam gibberish.
One method sites rely on for anti-spam detection are user reports. But since the humans only pay attention to the new content, the spam is hidden on the old pages, where web spiders index it, and then consider the entire site as spam.
So sites will close old comment forms to prevent spam bots from flooding old pages nobody looks at anymore.
There are a few other reasons people might do that, like on some sites people get into a heated back-and-forth that lasts beyond the 24 hours, where although nobody else visits that page anymore, 2 people are replying to each other for days or weeks later. By closing it, they're preventing people from doing that.
When any thread can be resurrected, it's like a Hydra that can grow a new head out of its ass, not just recently chopped neck!
:)
Trolls and spammers love to necropost. Trolls do it to annoy. Spammers do it because they are finding postings automatically based on keywords, and don't care about the date of what they are following up.
The quality of necro-postings is almost invariably very, very low.
Even when it is done honestly by a non-trolling user, someone who replies without looking at the date of what they are replying to demonstrates low intelligence, which raises the probability that they are going to post something of low quality.
http://meta.stackexchange.com/questions/148088/why-are-late-...
Here's the trap though: What happens if someone bumps an old thread from 2011? A lot of ppl assume that this is fresh content on the frontpage and will skip reading the date and might be misinformed. The probability that information will be current staggers in time.
A simple UX fix can warn the user that the thread is from 2011 or the thread has been "bumped".
It also works because of strong community moderation and a well thought out structure, e.g. bespoke software.
- It's much harder to keep a community engaged on what's happening now if they feel like they have to patrol comment threads for articles that mean a lot to them.
- Either it's worth saying now or it's not worth saying.
The most egregious form of this I've seen is Chromium bugs being left open but people being advised to create a new one if the existing one is too old.