I don't believe Harvard is choosing the wealthy kids and the legacies because they are most likely to succeed, I believe Harvard is choosing those people because they are most likely to have the highest ROI for the Harvard endowment & future donations. Someone from a low socio-economic status or otherwise marginalized background that has worked hard enough and smart enough by 17 or 18 to be a good candidate for admission at Harvard is an outstanding bet to have the determination to succeed later on in life.