I flagged this answer for moderator intervention and provided this reason:
This answer appears to be personal opinion, rather than a factual answer to a question.
The flag was declined and the moderator left this reason:
flags should not be used to indicate technical inaccuracies, or an altogether wrong answer
I did not flag this answer for intervention because of any perceived inaccuracy. The problem is that it isn't an answer. The general expectation for answers on this site is that they represent the actual, factual answer to a question. Being a reasonable or rational argument is not a basis for an answer. This doesn't mean that an "answer" that is opinion is poor quality, it is literally not an answer within the scope of this site.
We have an excellent meta post on what this site does for the world. The wisdom there is that we should downvote any "answer" that is personal opinion. However, since the answer in question has approximately 170 net-upvotes it is clear that this is not the solution we have decided to use.
So how should we handle these? If we can't trust the community to down vote them, and we can't rely on moderators to enforce the principles that the quality of the site depends on, what is the preferred course of action for dealing with opinionated non-answers?