I understand what you mean about locking a post, because people posting now will probably never reach the original poster. But I don't think there's any harm in generating a new conversation over an old topic.
I use the archive method, so I only see the new topics. If I post on something, I sometimes go back through my posts to see what kind of follow up there's been on a post. Otherwise, if I read every post, I'd be here 24/7!