This is wrong. As you move up the ladder to more and more intellectually demanding jobs, the median IQ gets higher and higher, even as the fraction of the population with such an IQ falls off exponentially. Yes, there are other important factors. And yes, if you have low conscientiousness then there will be diminishing returns going from high to very high IQ. But there is no threshold, and IQ remains the largest single predictive factor for success by far.