A moderator converting an answer into a comment is not the only way by which users without the commenting privilege can comment. The other ways are:
- Posting a "trivial answer" that is short and consists of an internal link to somewhere else on the same site, which the system will automatically convert to a comment
- Flagging to close a question as a duplicate, which leaves an automatic "Possible duplicate of" comment
If this request is implemented as you've currently proposed it, it would allow users without the required rep to post comments by just...intentionally triggering the "trivial answer" script, or flagging to close the question as a duplicate of some random question then retracting the flag.
You might think that it's rare for one of those scenarios to occur, but you'd be wrong. In the past, there existed a loophole where users without commenting privileges could post comments by triggering the "trivial answer" script, then editing the comment. SE felt it was important enough to plug that hole and prevent such users from editing those comments. (It was only then that SE decided to allow such users the ability to flag as duplicate; it previously required the ability to comment due to the automatic comment.)
Additionally, moderator conversions of answers are by themselves rare. Most non-answers are reviewed by normal community members who do not have this ability, and it's also rare for post authors to reply back to such comments.
Thus, while I agree that this feature would be useful under this specific scenario, it has too many potential pitfalls, and the scenario is pretty rare. In my opinion, the moderator guidance should be modified to say that the conversion feature should only be used for statements, not questions; the moderator should have left a comment about needing to attain the required privilege and subsequently deleted the answer.