Falling birth rates have little to do with child mortality and everything to do with child labor becoming less valuable due to legality as well as industrialization. Children became liabilities instead of assets to parents, parents had less kids. And the current problem is actually not enough people are going to be around to maintain the current economy of scale both in terms of production and consumption. The demographic cliff looming in Italy and China for instance will probably alleviate some of these environmental issues we are discussing, but at the cost of political instability.