Optimum is zero, because if you could wave a magic wand to magically make the amount of accidents zero, it would be a good thing to wave the wand.
The optimum is non-zero only if there are enough costs associated with making it zero.
That's why asking for optimum amount of fraud is misleading. It omits the costs. Once the costs are taken into account (i.e. it is clarified what the question means) the answer is obviously above zero.
"Optimum is zero, because if you could wave a magic wand to magically make the amount of accidents zero, it would be a good thing to wave the wand."
You are conflating optimum (highest value outcome for all variables) and ideal (highest value outcome for one variable). The ideal number of any bad thing is zero. I can't, off the top of my head, think of any bad thing for which the optimum number is zero. Extinction level events, perhaps.
The optimum is non-zero only if there are enough costs associated with making it zero.
That's why asking for optimum amount of fraud is misleading. It omits the costs. Once the costs are taken into account (i.e. it is clarified what the question means) the answer is obviously above zero.