Aside from the eugenics nature of it, this is an experiment we're gradually trying naturally. High-SES people are having children at later ages and are mostly

Aside from the eugenics nature of it, this is an experiment we're gradually trying naturally. High-SES people are having children at later ages and are mostly having children with each other. If there's selection effects to be reaped, we'll witness them. But I suspect we won't, because not enough people die before these later childbearing ages to affect things much. Neither do a great enough fraction die before said later children reach adulthood, and high-SES children have prospects of reproducing affected less by loss of a parent, anyways.