I think you’re conflating the terms optimal and ideal. The ideal amount of fraud in society is zero. The optimal amount of fraud in society is not defined, because optimization problems are always subject to a set of constraints.
So then we may ask: “what is the optimal amount of fraud in society such that the costs of legislation, education, and enforcement do not exceed X% of GDP?” and that is a different question. You might also throw technology and R&D in there because new tools make it easier to investigate fraud. Of course new technologies also open up new possibilities for fraud, so this is a very complicated exercise. But I think it’s fair to say that given any reasonable constraints, the optimal amount of fraud is nonzero.
So then we may ask: “what is the optimal amount of fraud in society such that the costs of legislation, education, and enforcement do not exceed X% of GDP?” and that is a different question. You might also throw technology and R&D in there because new tools make it easier to investigate fraud. Of course new technologies also open up new possibilities for fraud, so this is a very complicated exercise. But I think it’s fair to say that given any reasonable constraints, the optimal amount of fraud is nonzero.