Definitely not. Someone digging up an old thread is someone using the SEARCH function. This is A Good Thing, it helps prevent people asking the same question over and over. Maybe they have something useful to contribute to an old thread, maybe they need clarification on a point in that thread, maybe a link that used to work in that thread no longer works. It's definitely A Good Thing to be able to dig up old threads.
I'm far more likely to lock a topic that was already discussed in and old thread and put a link to the old thread in the new topic, and I've seen the other mods here do that as well. That would indicate a user that either didn't search, or has poor search skills, and we're just being helpful redirecting them to the already in progress discussion.