It seems quite likely that Silk Road was one of the safest ways to acquire these drugs. It seems likely to me that shutting down Silk Road would have two effects: reducing consumption by casual (non-addicted) recreational users who will not bother with less convenient and safe sources, and driving addicted users to less convenient and safe sources. You might argue that the first effect is slightly beneficial, but I think it is far outweighed by the harmfulness of the second effect.
> I've still got no real moral qualms about punishing someone who enriches themselves off so much harm to others.
This is just another way to rephrase the claim that Silk Road did net harm to others, which we were just debating.