Kids are overrated in cost by parents who are, basically, self-centered.   Cooking for four or even six is not really much more expensive than cooking for one.    You just buy smarter.    People without kids go to the grocery store shopping for meals, component costs which exceed eating in restaurants.   People with kids go shopping for food and they buy it in bulk.     Diapers can be expensive, of course, but poor people potty train earlier.   You see all kinds of poor families with kids potty trained at 18 months and all kinds of rich kids who aren't potty trained at 5.  So it's better for the environment to have more kids because in the end it will mean fewer diapers in a landfill.

Plus from a culture point of view, more kids are not only sustainable but essential.   If people in other cultures are breeding like rabbits and we are not, we will be overwhelmed.   That means our culture will go bye-bye and look what we would have missed:

King Lear - No one knows what this is about except a whole lot of siblings fighting, and many consider it a reason not to have so many kids, but it's Shakespeare, so we need it.

Chronicles of Amber - that guy had so many kids he had to start naming the girls things like Flora and Fauna.  The world needs kids named Fauna and Flora more than it needs clean air and cures for cancer.

The Waltons - "G'night John Boy ..."  and then ... what?  Crickets chirping?  No, you need more kids (Christmas bonus - watch the Waltons Homecoming, the 'pilot' for the series, though it was actually a movie that was so well liked it became a series, you will love it)

18 may be a little much.   Chris Rollins wrote about the uncanny valley in robotics and it applies to kids too.  Somewhere after 6 it is probably no larger charming and approaching creepy.

I should have just written my own article.  This comment is longer than your blog.