The optimal amount of airline catastrophes is non-zero.
The optimal amount of pollution is non-zero.
The optimal amount of [insert almost anything bad] is non-zero.
The optimal amount of pollution is non-zero.
The optimal amount of [insert almost anything bad] is non-zero.
The ideal amount of anything bad, is zero. But the optimal amount is going to be higher than that, given we don't have unlimited resources to spend.
The optimal amount of airline catastrophes is zero. It is also impossible.
Should all humans spend all of their effort and all of their resources to lower the amount of airline catastrophes? I believe that pretty much everyone finds it reasonable to say no.
Would you take a $10 10,000 mile flight across the world with a 1% chance of dying? How about a $100 flight with a .1% chance of dying?
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.
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.