That doesn’t work, on multiple levels. Existing is a prerequisite to wanting; you cannot want if you do not exist. But even if you ignore that, it’s not immediately ethical or unethical to do or not do something just because someone else wants it. But even if you ignore that too, there’s no way of knowing that child won’t be the next genocidal despot. And even if you ignore all that, you could never birth even a fraction of everyone who could ever be born, since those depend on genetics and you can never have every combination of human mating with the other half.