I do not think we should be overly worried about threads where the discussion has died down and no new replies are being made.
As we often have members say it's ideal to use the search button before making a post so as not to make a repetitive topic. So locking such threads would be counterproductive in my opinion, and users would have to make new topics to discuss the same issues.
The real concern are the threads which gets comments regularly with the same replies twisted in different ways.
But what do you think about those threads that clearly have been answered and no need of another answer whatsoever? As you said, the issue is that people keep on replying the same replies twisted in different ways. If the OP just locked the thread right away after he/she got the answer he/she needed, then spam replies wouldn't get a chance to even be made.
The issue of locking threads is not a straight case, and it's up for debate when a post should be locked.
That's why this is feature servers a truly optional action to do. If the OP doesn't want the thread to be locked, then he/she just need to decline the notification.