Unlike forums and other sites, this site focuses around the idea of questions, answers and comments, so it is preferred that answers are not used as comments.
I guess that the first part of that is seen as a misconception. The correct/"official" take on this is "this site focuses around the idea of questions and answers". No comments at this stage. Comments do make this more like forum, especially if "abused". But comments are volatile, ephemeral and subject to deletion at any time.
This proposal of lowering the comment privilege threshold would primarily increase the amount of comments, among that increase would also be bad comments. Bad comments are chatty, thanks, not about improving the the post they are under. That happens. I like some of those a lot since they provide a channel of communication not otherwise present. But it is frowned upon and contrary to the site's design. I guess a lowered threshold would likely tend to increase the problem. That does not mean I would be totally against lowering thresholds.
New users typing text into a wrong box is one problem, old users doing the same under different circumstances the other side of the same medal. It seems to me that new users that stick around are much quicker learners in that regard that "do not put comments into answers" is actually followed than older users that still "answer in comments".
I do not have that much insight on absolute numbers. Therefore I cannot estimate reliably how disruptive the problem you identify really is. But the current solution to it is flag the post for deletion or conversion into a proper comment. That's work for the mods. They seem swift about it.
If you strive to be really helpful to the new user and comment on "answers that are really just comments" yourself, teaching them the ropes, then your struggle with repetitive typing might e reduced by having a text snippet prepared for that occasion. That reduces the messiness quite a bit. It's then not typing but copying that bit you already wrote here.
