Well, considering the original thread and last reply were posted nearly 2 years ago
(until yesterday), I think this illustrates very well why something should be done about very old threads that have been inactive for some time.
It's like having a 5 hour conversation with someone over a variety of topics when suddenly the person you're speaking with makes a reply to something you said in the first 15 minutes. Where did that come from? Haven't we moved on by now? What does that have to do with anything we've been talking about for the past 4.75 hours? (Feel free to do your own mathematical conversions of years rather than hours.)
Maybe when I wrote the original topic, I was really mad about something (such as what was then a "recent" necro-spammer trend). But now, two years later, that "trend" isn't much of a trend, so the whole conversation is pointless because it no longer reflects the current reality. Or maybe I've had a change of heart since then. Maybe I'm a completely different person now than I was then.
The point is, when someone resurrects a really old thread, and it's been so long that nobody remembers it or perhaps they are a "new" member (within the past two years since the thread was last discussed) and have never seen it before, people start jumping in like it's a fresh conversation because they don't notice the date, when really it's been dead in the water for years.
To the person doing the resurrecting, it's a bit like coming up with the perfect comeback two weeks later. They really want to say it, but they know it's no longer relevant since so much time has passed, so they don't say it. Unfortunately, because the date is so easy to miss, people often don't have the perspective of time to keep themselves from realizing that though what they have to say is perhaps completely valid and would have added to the conversation at the time, the conversation is dead and not worth bringing back up again, especially in a manner as if it never ended.
In normal conversation, the way to go back to an older topic is to first remind a person that they had it (or somehow transition the person back to that subject) and then make your point. e.g. "Hey, remember when we were talking about such-and-such? Well I've been thinking about it and. . ."
I don't know if locking topics is the correct solution. At the time it seemed like a good idea. But now that I'm two years wiser (that's debatable, I'm sure) it still seems to me there ought to be some notification to people that they're reading a really old thread, or that the new post(s) by someone else were made to a really old thread. And definitely some sort of notification when people are replying to a really old thread (even if a reply has recently been posted, IMO). Kind of like that one that says "Hey, while you were typing, someone else replied to the thread." It's all an effort to help you make sure your point is "up to date" and as relevant as possible. You can still ignore it and post it exactly as you had it, or you can read the reply and make any necessary adjustments to your response.
I guess that's all I have to say about that. For now anyway.