I would mod them in accordance with P&E policy to keep them on the given topic. More wide-ranging discussions are also possible in P&E. I started one myself. No one has been modded there for being off-topic AFAIK.
I do believe in some amount of organic shift in the conversation as a thread progresses, and I’ve said so before. But the point remains, IMHO, that someone joining a thread should not have to wonder WTF the discussion they wanted to participate in has become, where it has little to do with the original topic.