Starting a new thread isn't really an option. There are essentially two avenues:
1- Start the topic anew and retread a bunch of stuff already discussed, which isn't going to work well for the people who were already involved in the discussion.
2- Start a new topic with a link to the old thread telling people, "Go read this thread to get up to speed and then come over here to continue the discussion," which for obvious reasons won't really work.
As for people locking threads that they felt got "out of hand", well, I can't lock any other threads that I felt got out of hand. Why should I be able to lock my own. When I start a thread I have no better idea how it's going to turn out than when I read the first post in someone else's thread. I don't see how I'm any better qualified to lock a thread I started than one someone else started. I can read through either one and decide whether I personally believe that the thread got "out of hand", but that doesn't mean I can, or should be able to lock any thread I believe got out of hand.
Seriously, if the thread had been reported to any moderator, the moderator would have read it and been like, "Eh? People are just talking here. Not a single uncivil thing has been said (except for when the person who locked the thread called everyone "economist freaks", but I don't think anyone took that personally, and it would be pretty strange for a person to break a forum rule just to give himself personal justification for locking a thread).
Anyway, I just don't see what use there is in letting people lock their own threads. Moderators are in a much better, objective postition to do this. Locking a thread affects every person who was posting in there, not just the OP. Posting a new topic for discussion with the caveat that "You can post whatever you want here in my thread, so long as I agree with what you have to say," is pretty lame. If we were sitting around a coffee shop having the exact same discussion I couldn't just put in my last word and then announce, "Okay everyone, you no longer have the ability to discuss this issue, even after I leave. This is a power I weild merely because I am the one who asked the initial question that got this conversation going. I therefore own the conversation."