Well, the optimal quantity for many things is actually zero.

It's the edge cases-- the things where it's really hard to get rid of and we get some useful benefit from the related activity-- where it's nonzero.

Of course, those edge cases are our biggest world problems-- for example, pollution, because pollution abatement is hard and the industry and commerce that produces pollution is beneficial... or fraud, because fraud protection is hard and the industry and commerce that provides opportunity for fraud is beneficial.