Success is an abstraction, not a measure (or even unambiguously measurable.)
Right, but IQ is still one of the best predictors of success.
But what is the best predictor of IQ?
Off-the-cuff, I'd probably say either a stable and supportive family environment and/or above-average family wealth, in which case one could just cut out the IQ middleman and say those are the best predictors.