[mlyle]

> I guess this can all fit within a “marginal cost” explanation though.

Yes, but it undermines the first point, a bit. There's costs-- direct and social costs-- to making transactions hard; so perhaps optimal for a society is still not 0.

Also, there's nothing to say that the amount of fraud is stable and that we can't find a world where we have better mechanisms to reduce it for the same cost. (Improved technology, legal structures, norms, etc).