Even one upvoted zero score answer means 100% positive response. Isn't it great?
No, it isn't. Making a user who has made one post that got a single pity upvote seem like a more able contributor than someone who has made thousands of posts with an average of 10 upvotes per-post is... well, it kind of speaks for itself. Mediocrity is not something we want to encourage.
Reputation may not be a perfect system, but it's a lot better at showing who is posting content that is genuinely valued by the community and who has yet to do so. We don't want to hand out "participation awards"; we want this stuff to mean something.
And your "positive response" doesn't mean anything. It doesn't mean anything about the content a user posts. It biases towards people with fewer contributions than those with more. It doesn't properly discourage negative contributions, since those don't get properly recorded. It's just a bad system if you care about quality.
We don't want to encourage people to stay. We want to encourage skilled users to stay.