The more negative SEO that happens, the more data that gives Google. Overtime, the negative SEO sources will be over-saturated, offering no benefit for either positive or negative SEO.
Driving down the positive and negative value of links makes all SEO link building efforts consequently worthless. Which in turns increases the importance of other ranking factors.
This is a natural evolution, in tiny tiny steps Google is nudging people into playing it's game - producing websites that have content. SEOers are slowly falling into line building private link networks, paying people to write content, afraid to spin/rehash existing content because it might leave a fingerprint Google can detect.
It's fascinating watching.