Do you think genetically stupid people should not have kids since the kids will also be like that and having a child would just be adding misery to it's life since it would never be able to be successful or achieve anything. S/he would spend all their life being inferior to other and it would just be a lifetime of pain.
When I was a young man, I knew someone who was, in the phrase that might have been used at that time, "mildly retarded." He was married. And he understood his condition. And he struck me as a happy man. He certainly wasn't leading a life of misery. In the neighborhood where I now live, there is a young man who is even more intellectually challenged. I doubt that he understands his condition. But he does not strike me as unhappy at all. To be sure, he lives a simple life. And no: he couldn't live on his own. And he also won't have "accomplishments" in the sense you have in mind. But near as I can tell, he's not miserable at all. He's happy. In his case, I don't think marriage is an issue. But the larger point is the important one: intellectual ability and happiness are quite different things. There are sad, miserable geniuses and thriving, happy people whose IQ scores are well below 100. So what I'm saying is that I don't accept the premise of your question.