It's interesting that none of this, or even the concept "downvote" is mentioned in the tour for new users... superuser.com/tour . The concept of comments is, but how they relate to downvotes is not. Granted, the explicit invitation to comment when you click "downvote" is there...

I can agree that for questions without accepted answers, but highly upvoted answers, it can still be quite clear the answer is useful. So the argument for a downvote would be weaker here. Although, when using the search bar, and seeing a list of results, the usefulness would be higher if the question is actually marked green as answered. That is an aspect that is lost with the current approach of leaving answers unaccepted. So I'm undecided on your first point, and agree with your second point.

I'd consider the questions with a small amount of upvotes the ideal target for this approach. If 2-3 users downvote it, it's gone... "fake" unanswered questions with a lot of upvotes are the real problem....

Ah yes. Indeed.