When someone posts a short answer, but it's not a comment, it shouldn't be converted to a comment. If it's too short to be useful, it should be downvoted or, if it's very low quality, perhaps even deleted. (If these choices aren't palatable, adding a post notice asking for more detail is always another option.)
The way things are, people are encouraged not to post short answers, even if a short answer is all that's necessary. I know I've avoided answering questions in the past because I couldn't think of enough filler to pad out an answer to the expected length. And I've seen other people post valid answers as comments, keeping in mind that they're "too short" to be acceptable on ELL, even if they're better than the actual "answers" they're competing with.
Worse, users might post answers as answers, then have them converted to comments which can't be downvoted, when what we really need to do is mark the answer as unhelpful by voting. The resulting comment-answers appear above all the potentially helpful real answers.