I think perhaps the broader point is that it's ok for discussion in places like news & rumours to be much more free-wheeling and discursive.
I get that the same old posters can go back over old ground, but against that, when I'm chatting amongst friends I have never just stuck to a single topic at a time, discussed that, decided it was time never to speak of it again and then start a whole new conversation about another topic.
There's a significant middle ground to the current approach and a Reddit style morass; I can't help but there is a little bit too much "rules control the fun" going on.
As to suggesting that the discussion should automatically follow the closure and movement of posts, perhaps it's worthwhile for the moderation team to seek to understand why that isn't happening, and the auto-response not to be to blame the users...
These are two fantastic points.
Conversations meander and veer to this and that side - it's what they do and what is inherent to verbal interaction. Deciding when that veering away has gone too far is crucial, but has been handled far too rigorously in the last few months in my opinion. Pruning and cutting into anything that isn't 99.99999% related to the OP topic at hand just stifles conversation and is counterproductive. If a side-conversation cannot be engaged - within reasonable limits of topicality and behaviour - within a bigger conversation, then there is no other point for the N&R forum to exist as it does right now.
What has also become apparent - from private conversations and public discourse on the matter - is that the mods, or at least some of them, view the closure of an N&R thread and the shifting of topics to the relevant sub-forums as a "shifting" or "relocating" of the conversation - or rather, the conversation is just being moved but apart from that continues onwards as before. This isn't quite correct, really. The conversation is being interrupted and stopped, and then has to later on be unpacked again by participants and mods alike so that it can be picked up again. This is tedious and stifling to the flow of interaction and communication. Imagine talking about something in the kitchen, someone comes in and tells you "Not in here, do that in room x" and so now you get up - mid-conversation - relocate, sit down and then try to pick things up again. Except, by that point one usually has lost interest or connection to the prior topic, but is still frustrated. We've all been there.
As Alfred_the_great says, there are a lot of reasons as to why traction barely picks up again in subforums. Lack of exposure to the average user, frustration over topic pruning, frustration over interruption of conversations, general annoyance of not really knowing how much off-topic is too much, lack of cross-faction pollination within the conversation (Some ork players might be interested in conversing on the new release with wolves players and vice versa for ex.), selective interest in specific sub forums etc.
I do appreciate and understand that moderating any subforum is a big undertaking and that it takes a lot of time, nerves and work - but pushing this onto the community à la "Well they COULD talk about that stuff there - they just aren't doing this for whatever reason" just seems like an attempt to distance oneself from something that is clearly a two-way street.