Given that comments are not forever, when someone comments on one of my questions or answers looking for a clarification, I should be going back and editing my post instead of responding with another comment?
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It would be preferable to edit your question and add any significant clarifications there. Comments, as you note, are ephemeral and they aren't really as visible as the question, it's not uncommon for a clarifying comment to be completely out of sight if there are lots of upvoted comments.
If you want you could respond to the commenter afterwards, telling them that you've updated the question.