While I don't have any expertise here, one of the things I hate most in life is "drive-by downvoters" who have nothing constructive to say and only stick around long enough to downvote. So mainly I'm just here to say sorry about that.
I don't see the case for the "average person" having positive externality. In my view, most people are close to neutral, a minority leave the world worse than they found it, and another minority make the world better. I suppose that whether people make the world better or worse probably depends mostly on culture, as most people are sheep and will work to fit in with their culture; the goodness or badness of the effects of that culture on the world won't make a lot of difference to their choice to conform. This explains the difference between an average person who grows up in, say, Germany in the 1940s and becomes a Nazi soldier, versus one who grows up in, say, Canada in the 2020s and becomes a plumber. It's not genetics driving this sort of thing, at any rate.
I can't answer your headline question, but would be curious to hear others' thoughts on the matter. I will say that while the expected value of having a child may be high, the risk of any kind of defects is very scary in a way that is out of proportion to the EV, since, if you get unlucky, you will probably spend >18 years with a child that you love, of course, but who probably won't be able to achieve what you hoped.