I don't think I'll be doing any research on this suggestion, though I appreciate that you made it. The reason for this is because I just don't understand what could be gained from having it. I tried to think through some potential benefits as best I could, and I came up dry. You mention "not reviving old topics when we don't need to," but really, I would think that the answer for that is simply not to revive old topics when you don't need to. The rules have nothing against thread necromancy when a member has something good to add to a topic, and in fact, a few years ago we did it intentionally for a week.
We don't really get too bent out of shape if someone posts a new thread instead of reviving an old one; our rule of thumb is generally that if there's a similar thread on the first page of the forum you're in, that's pretty silly - but by and large we'll either just let it go or we'll merge threads when we know it's appropriate.
I don't think that posting new threads versus reviving old ones will make a big difference in the number of posts made at CoN, either; if someone has already answered a "what's your favorite esper?" topic before, for example, simply seeing a new person asking the same question isn't necessarily going to make them re-think the topic. And, even if it does work that way, that doesn't mean that I won't eventually come through and merge the threads anyway.
I also think that if a thread was automatically locked, over time you'd end up with newer users coming to the site, seeing that most threads are not available to them, and instead of that fact making them more likely to post new threads, it would make them more likely to assume that the site is dead and just walk away.
I'm just spitballing here, of course, as to what problem you're looking to solve. If you'd like to explain it further, or provide examples of other places where such a tactic has been beneficial to forum activity (maybe this is what you mean by saying you're going to test it?), please feel free to do so. This thread isn't getting locked, so you'll have plenty of opportunity.
