Having a child is type of experience and commitment that has no parallel in any domain of life. It shapes you in certain ways if you are devoted to it. I want to be that type of person because I recognize some qualities that I admire (like being way less self-centric).
My body is designed so that it can do this. Hence, not having children is against the design. Its like using oven as newspaper holder - nothing wrong with that, but using oven by design is something else which you can't really describe to non-oven people who never tasted cooked dishes. However, it takes time and there are number of responsibilities involved and there are huge number of people who think its just waste of time when they can simply go burger king or avoid eating at all until they must.
That is not nothing.
Yes, it can go bad, like anything else in life. You can die in fire using oven too.
I think this is rational explanation.
So called "Socratic procreation" (in short, where your procreate via recognized work/ideas instead of sexual act) on the other hand, is IMO meaningless (and maybe even compensation) for the very reasons mentioned in the blog post.
That said, I do want to have a health span (not life span) as long as possible and that has nothing to do with all of this. I can freeze my head in a jar but that doesn't really count as health (while it could be counted as life).