"Academic success" is required to enter many highly paid occupations: law, medicine, finance, etc. So I would disagree it is a useless indicator, it is directly related to one's future income.
If you measure success by one's income, which is a good and clearly defined though still arbitrary indicator, I see no reason whatsoever to expect it would depend on IQ. People with high IQ are often less competitive and often question the need to maximize their income, which is obviously detrimental to such income.