It might be possible to handle this as part of T265750: Make it easy to "outdent" by changing the colons/list level.

I was solving the same problem in Convenient-Discussions and collecting feedback from users a couple of years ago. My conclusion as a result of this discussion was the following:

1. Comments at the 0th level (without indentation) split a section into several parts: replies to the original post and replies to other comments at the 0th level.
2. Despite the fact that comments at the 0th level do reply to the original post at least in the sense that they are placed in the section created by that post[1], they create an additional "root" in the hierarchy.
3. While doing that, they nevertheless do not establish a prominent enough boundary between these parts of the section.
4. They also violate a reasonable enough expectation of both posters and readers that new replies to the original post go at the end of the section[2].
5. We already have a means to split a section into several parts: subsections.
6. Unlike bare 0-level comments in the middle of a section, they do establish a prominent enough boundary between parts of a section: the headline, in which the author can articulate the reason why they chose to deviate from the subject suggested by the original poster.
7. As a consequence of the above, creating a subsection with a clear headline should always be preferred to adding a 0-level comment. A discussion tool should not encourage adding 0-level comments.

So I sticked to this reasoning. I think at some point I did witness an exception to it (can't remember what it was unfortunately) which led me to allow creating a subsection without specifying a headline, which in practice created a 0-level comment as suggested by this task. But that's basically an undocumented workaround, and I haven't met any evidence to suggest it should be something bigger.

[1] When a section is not created by a post, i.e. it's a section without a leading post (e.g. pre-created sections with votes or comments on a known subject), comments in such a section are usually all indented.
[2] Or, if there are subsections, at the end of the first chunk of a section, before the first subsection.