Your salary was the exact kind of thing that needs to be balanced against the cost of fraud; if it was larger than the amount of fraud you prevented, then the company would have been better off just accepting the fraud as a cost of doing business. The closer you get to zero fraud, the more expensive it becomes to reduce it further (and the more likely your countermeasures will negatively impact the business in other ways), so there definitely is an "optimal" balance to be struck between fraud and preventive measures.
You don't live in an optimal world so what use is it talking about? Are you also doing your business calculations using frictionless spherical cows in a vacuum?
Simply put every living system on the planet earth has some amount of parasites. To have no parasites at all would require massive amounts of energy by the host to ensure said parasites don't exist. If the host can spend a much smaller amount of energy and ensure that 99% of parasites don't exist that is optimal thereby negating your original premise.
I worked in retail loss prevention, what is acceptable isn’t optimal.