That's completely irrational. So one of these kids might do something good?! Let's have more! In fact why wait for couples do have kids, just IVF then with surrogates and create extra people, in case one of them cures cancer (then we will have even more people!)
People worried about overpopulation shouldn't have kids because it's bad to be legitimately worried about actual problems that will affect the quality of life for humans forever. Okay that makes sense.