The basic immorality/pointlessness of the war on drugs aside, I don't know how you can assert this: it's not like there's a chain of provenance, and there's no particular guarantee that whatever grade of pure drugs was sold on Silk Road is the same purity that ended up in peoples' bodies.
My understanding of the Silk Road case is that, at its peak, it was servicing a significant portion of the international drug market. The dimensions of that market include adulteration; Silk Road almost certainly didn't change that.