I think we should leave them be, because the downvotes make it clear that it is wrong, and showing that something is wrong is useful information.
For example, if I read an answer that retells "the airfoil myth", and it has -17 downvotes, I will assume that the airfoil myth is wrong, which is correct. I actually learned something.
If the answer was deleted, then maybe the idea of the airfoil myth may creep into my mind some day from other less reputable sources.
A biology point of view: Being exposed to harmful answers in a controlled environment, where they are downvoted, shows you they are wrong and thus gives you an "immunity" to them, that remains after you leave this site. Very much like a vaccine.
A geometry point of view: Ideally, positive votes show how correct an answer is. Then, negative votes show how incorrect it is. If an answer is a 2-D vector pointing towards an idea, then a negative count inverts the direction, pointing away from a wrong idea.
An engineering point of view: Squeezing some right knowledge even from wrong answers is efficient at the least. Deleted answers give nothing at all. Plus moderators don't have to be deleting extra stuff. Unless storage space becomes crucial, then yeah, delete.
Edit: the original answer used "chocolate electrons" as an example, hence the comments.