I my opinion, it's to early to lock a question when moving off-topic comments to the chat for the first time.
It's understandable when it repeats two or three times... and it certainly did in the past (that I'm pretty sure did not end with locking) but the first time should definitely leave the question open.
This post is based on this question that has suffered such a premature locking.
This post has been locked due to the high amount of off-topic comments generated. For extended discussions, please use chat.
Their amount wasn't that high anyway and not that different from other popular questions.
I'd be great if a ♦ could undo this.