It's a difficult balance. Moderation on here is fairly light touch and we allow a certain amount of banter and single off topic posts or mixed on/off topic within a post.
It's usually only when a large tangent starts that we consider taking action. Sometimes that means moving the tangent to start a new thread or into an existing one.
We also then get some people unhappy about threads being allowed to stray off topic and some about them being over-moderating. That's life!
Some threads are more difficult to moderate. As an example, this "I'm out" thread covers a wide range of topics that could be moved into other topics, but generally the posts are all prompted by cancellations. A side discussion about a Vantage or F-Type as a candidate instead of a cancelled Emira could go into the "Emira vs the competition" sub-forum or stay here.
The "issues found on production cars" is another tricky one, as we want it to be about field reports from owners but inevitably it will lead into some people trying to help solve the issues and others who want to express concern about what is being reported.
All the moderators are doing this in their spare time and we all try to get the right balance. We're happy to discuss the general approach to moderation and hear feedback about what we could do differently. A topic for a new thread I suggest...