I still think that it's silly, because the common justification given for the position is highly suspect and borderline sophistry, and is, I suspect, not the causal reason for the values it purports to justify.

I'm curious what you think the causal justification is. I'm not a fan of imputing motives to people I disagree with rather than dealing with their arguments but one can't help but notice that Heinrich Heine was paralyzed, blind and in constant pain for the last decade of his life. Moreover, his religious beliefs prevented him from committing suicide. In that context, antinatalism just in regards to one's own life seems to make some sense. Thus one might think of antinatalism as arising in part from Other Optimizing