There are different kinds of discussions here on the forum. If the thread is something relating to a question of which has been answered already, it can be locked and moderators who notices such usually locks them. A wise op, can consider locking his own thread ones he gets the answer he requires. Also, there are other discussions where ideas keeps coming in and the discussion keeps going on. Those kind are what should be left open.
It would be somehow, to open a board and see so many locked threads just because the moderator feels that there's nothing more to discuss. Except for situations where users are now spamming or causing commotions.
Yes. Threads that have been successfully answered already and OP is now satisfied with the answers given, then he should be responsible to lock his own thread. His question, his own time when he will be satisfied with the varied answers coming in. However, in order to avoid spamming or having more answers that are already going into off-topic, it’s best to lock the thread as soon as answers are already considered the best thoughts and ideas that any reputable member can share.