I mean the reason they cite for locking them down is a really weak one.
I'm still waiting for somebody to point out exactly how an old, necroed thread makes it "look bad" or "confusing", as opposed to opening new ones (which actually does make it confusing).
Generally it's a thread that no longer receives comments for significant span of time potentially making it outdated, like 2-3 months or more (or worse, years). The severity of the necro is determined and assayed by whoever is moderating at the time. What they consider a necro I guess is up their random discretion.
¯\_(ツ)_/¯
But take the "elves need an upgrade" thread. It's still relevant now, and as Xafnir pointed out, it actually creates more confusion by creating a new thread and linking the old one/quoting parts of the old one in it.
I didn't see the thread when it was created, but after it was necroed I saw it, so I wanted to add my 50 cents, but then I noticed it was locked
The real question is why the rule is in place in the first place. What it comes down to is that it seems to be a common practice on internet. So the rule is made and enforced, in the absence of why it actually existed in the first place, and instead is so because "everywhere else does it too", and that is the limit of their thinking about it. Personally, I think it would be better if this policy was cancelled.
But that doesn't happen everywhere. On most professional forums and communities, what you DON'T want to do is make a new thread (or ticket) for the same thing over and over again. It's better to necro an old one, than to create a new one (ofc, if it's been closed, you should create a new one).
Anybody that works in IT, dev, ops, etc. and uses something like Jira can vouch for this