Perhaps we could look at the downvote system as a courtesy we all provide (at no cost) that is used to the communities advantage?
I think there are people here that do this. There are also a large number that don't purely because of the incentive associated with upvoting.
Again that's why I like to think of it as an irrelevant vote that still carries some precedence to the community as a whole.
Perhaps this is what you want, but the message being sent is: upvoting is more important to steemit than downvoting. This seems also the case from a usability perspective: upvoting is at the end of a post and clearly associated with its incentive; downvoting is at the top right and looks like a flag.
I hope I don't sound too insensitive. I'm not sure that rewarding downvoting is the answer, but the current situation doesn't fill me with confidence that we'll achieve what we want: that quality content is made visible to Steemers and that plagarised, fake, and otherwise poor content is not.