TL;DR: I agree with you that such a feature might be handy, but it should be taken care that we don't get low-quality answers as a result.
Note that comments are mostly written because the commenter did not want to write an answer in the first place. Those two are different concepts, and while turning a (not-an-)answer into a comment works very easily if it's of bad quality or not particularly helpful, the other way 'round is – in my opinion – not a good idea.
A good answer should look completely different from a mere comment. This is because comments miss a few things:
- lack of formatting (what about code markup?)
- lack of possible submit length (600 characters isn't that much)
- lack of research from the person posting the comment
Now, if you turn such a comment into an answer, it might be helpful, but still just intended to be a comment and therefore maybe of really low quality. Of course, this doesn't always have to be the case, but in your example above it did. One link only is the answer?
Anyway, in such a case, the general practice is:
- Ask the commenter to write a proper answer. After all, they deserve the reputation from it and maybe just weren't informed that they solved the problem. In the above case, @paolo was never mentioned in a follow-up comment and therefore didn't get any notification.
- If they fail to do so, ask the OP to answer their own question. Maybe there were some additional steps involved that haven't been mentioned yet.
- If this doesn't happen, answer it yourself. Stealing comments is fine with proper attribution (and, additionally, turning it into Community Wiki as you did)