This is not fair to the people responding to a topic, IMO. People who reply to topics should know that if they are on-topic and they follow the rules, they will not be censored. Also, it is very annoying for reply-writers when they write long, detailed responses and then the topics become off-topic and useless.
Well, it's your forum, but I've never seen OPs suddenly deciding replies to the first post to be off topic and complaining to moderators (who wouldn't then .. check?).
It's far more common to see posters in a thread getting irritated when moderators who are not part of the actual discussion go around and deleting posts.
(Not here, yet, but that's a worry with the change you outlined)