Do moderators have the ability to create an answer from a comment? If so, I think this could be used to transform comments into an answer (keeping the original owner) even if the user doesn't migrate the question.
If moderators don't have the ability to do this, maybe the community can do it on their own, provided that they indicate they are not the owners of the post (in a way the license permits).
What about moving those comments to chat? This would clean things up and be an incentive for users to post their helpful ideas once again as an answer. Still, the comments would be available in a chat room if someone wants to look at it.
Edit to address the plagiarism issue:
Disclaimer: I am not a lawyer, so I cannot give legal advice. The following is my interpretation of the linked material. I cannot guarantee this interpretation is correct.
The Section "3. Subscriber Content" of the Stack Exchange Terms of Service reads as follows:
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license.
The footer of every pages indicates this as well, it reads:
user contributions licensed under cc by-sa 3.0 with attribution required.
The description site of the CC BY-SA 3.0 license (https://creativecommons.org/licenses/by-sa/3.0/) states that everybody is free to share and adapt the material, as long as the following terms are met:
- The material is distributed under the same license
- Appropriate credit is given
- The license is linked
- It is indicated if changes were made
As long as those criteria are met when copying content from a comment to an answer, I believe copying is completely legal and the original poster can do nothing to prevent this.
The respect and courtesy issue:
I understand that it is disrespectful to copy content from someone without proper attribution, however I think the attribution requirements of the content license are enough to fix that issue.
What might be seen as immoral is that the converter will gain (or lose) reputation for an answer he didn't made.
To fix this issue, I think the best idea is to make such answers a Community Wiki. When an answer is a community wiki, the author is featured slightly less prominent and voting doesn't affect the poster's reputation.