Tbh great threads always have little side convos from time to time while keeping on one central topic. It's completely natural and people shouldn't get all emotional about it unless it's absolutely egregious.
Arbitrarily locking a thread because of the natural flow of the convo is ridiculous and turns people off of any forum in a big way.
One of the things I admire most about our moderators over at the Mensa forums in my 9000+ posts there is their hands off approach, only involving themselves if something becomes blatantly out of hand. Even then there is a mod chat board where we all