By my observation IQ correlates to anxiety disorders so often people with high IQs tend adopt risk adverse behaviors to minimize that anxiety and that undermines opportunities for success. People with really high IQs tend to end up on the spectrum and regularly burnout. Extremely gifted youngsters tend to end up with similar and often pretty severe health problems. So while I do agree that in general past success is a good indicator for future success and IQ is not the be all and end all, I would suggest that many high IQ people can be properly medicated to lead much more productive lives than currently do. And this difference would explain much of the residual lack of expected success.