In this case (where the original commenter is long gone/ignores suggestions to post as a full answer) the best thing to do is to post an answer yourself, using the comment as the basis for your answer. It would be usual to mark such an answer as community wiki if it is largely based on the comment rather than your own work.
Of course if the original commenter is still on stack exchange you can @reply them suggesting their comment would make a good answer.
Why I would be against comment conversion
The usual reason for posting a semi-answer as a comment is because the commenter was unsure of the answer and just making a suggestion or they felt the comment wasn't sufficiently detailed to be an answer. In both these cases a conversion to answer is non desirable (as improvement should be made) and unfair to the commenter, who made a conscious choice not to post an answer.
I can imagine a great many people being annoyed at being downvoted for a low quality answer that they never actually posted.
