I think it's more complicated than grades, IQ, or test scores. I know plenty of people that had mediocre or terrible grades, but had many successes in life.

How do you define success? ability to support yourself? Retire at 40? support children?

It also has to do with: drug/alcohol addiction/consumption, personal choices and life decisions (If you have children before you are married/can support them, you will most likely), and if you actually graduate from high school (grades reflect this, but you are pretty much destined to low-paying wages/jobs with no high school diploma in the US).

"Colleges and employers interested in predicting the success of applicants would do better to look at a student’s grades, which measure personality traits, like grit and attention to detail, more effectively than IQ and SAT tests"

High grades are a great predictor of high parent involvement and a knack for succeeding in a rigid education system. They might as well say colleges predicting long-term success should look at economic status of applicants.