The amount of discussion sprawl for a single question was in my opinion excessive.
There were 2 questions each with comment threads (What map scales have you found useful? (closed), Do the map scales suggested in the DMG tie into any rules? (with 10 deleted comments)) and then 2 meta questions also with discussion (How to ask about map scales?, Will this question about mapping not get put on hold as too opinion-based?).
As a result, instead of continuing comment discussion, I re-linked the latest meta question and deleted the rest.
If they choose not to engage in meta (and you don't really know they're not, they posted the meta and can read your replies, they don't have to turn it into a discussion), then you vote your conscience, you don't sprawl the discussion further.
Related to the specific area of concern for the original question, the community is in my opinion being excessively overzealous about identifying questions as designer-reasons questions. Someone explicitly asking for designer reasons is a designer-reasons question. Or just bellyaching about "Why did they do it this way I loved descending AC!?!". But it's entirely reasonable to explore, in a complex ruleset, why something might be the way it is. This is not a problem with the question, it's a problem with answers. If there is not an answer within the ruleset, then don't answer, and it's not a designer-reasons question.
This isn't just about that question, it's been happening a good bit lately and the mod team in general isn't very happy about it. And it's not just about overzealous identification of questions as designer-reasons. There's been a weird uptick in a particularly legalistic approach to community closure and activity lately which fails to recognize the organic nature of the community. We do kinda want to allow as much content on the site as we can that's not problematic. There's not always a "hard rule" and we expect people to use their judgement.
We certainly do, which is why there's not always some black-letter law underlying our actions and guidance to the community. This is an example of that kind of time. As @sevensideddie points out well in his answer, "it depends." Sometimes deleting comments, sometimes moving to chat, sometimes posting on meta, sometimes doing nothing is the right thing to do. In this case, the combination of discussion sprawl, on a topic that seems to need some guidance in general in the community, resulted in a specific set of actions (comment deletion and comment guidance to a) use meta and b) lay off the over-diagnosing of designer-reasons).
A similar situation in the future might indeed generate different actions. That is a feature, not a bug.