So far, there are two questions in this comment thread that I think are highly relevant to establishing a good answer to this question. I believe the follow-up questions should be included because (1) they're highly depended on this question, and (2) they would help determine what makes a good answer. In contrast, @GrahamMiln has recommended both of them be separate questions. Can we get a reference or an opinion from a third party to establish what the right approach would be?
This is an interesting case, especially because the question itself changed over time based on the things the OP learned (visible both in the partially deleted comments and on the answer the OP posted themselves), and not even the answer posted by the OP answers the question as written. So I'm not sure this example qualifies for a canonical answer regarding the "when should a new question be asked", it rather would qualify for a "how can we better help users to write good questions" discussion. The asker in this case didn't express very clear what they actually wanted to accomplish so the whole Q&A is rather a mess.
As general rules regarding the need for new questions based on comments I use the following
- If a comment raises a totally different topic (e.g. a comment on a Q/A about a specific functionality in Numbers asks about how to make sure one always uses the most recent Numbers application on all platforms): a new question is always the way to go
- If a comment points out a minor point missed in the answer (or maybe a part of the question which was not covered): by all means extend your answer
- If a comment makes it obvious that the question was asked incompletely, or crucial points have been missing: ask the OP to update the question (or update it yourself) and update the answer
In the case here, the followup questions in the comments ("Would copying Spotlight files be easier and faster than letting them be generated?", "How can I stop Spotlight so it doesn't mess meanwhile?", "What about caffeinateing the process?" and "When I try to use -E to sync to a drive on my Raspberry Pi") raise new topics not covered in the original question ("the idea is to mimic what I usually do manually with Finder") so recommending new questions for this is totally fine.
One might argue that the first set (Spotlight and caffeinateing) might be included into the answer if the question would be updated accordingly (neither Spotlight nor caffeinateing mimic Finder, so some editing would have been required here for sure).
So, to summarize (TL;DR version)
- Extension/clarifications of a written question which do not change the scope of the question as such should be added to the question (at least as long as existing answers don't get invalidated by it)
- Unrelated topics or things vastly extending the written question should lead to new questions instead
- Also, there definitely is no obligation to include comments not focused on clarifying the answer given. If people feel strongly about a topic they can and should ask a new question and not hijack existing answers (or question).