It's the dunk tank.
This is where you come to post big-brained hot takes by chuds, libs, or even fellow leftists, and tear them to itty-bitty pieces with precision dunkstrikes.
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What's the implied (final) solution to this extremely concerning situation, bucko? 
It's also a view that does not match reality. The material conditions are much worse in most nations in the global south and raising kids is more difficult, yet people in these countries are still having lots of children. In fact, one might obverse that the opposite is true, the more wealthier countries and societies are having the least amount of kids. So "improving the material conditions" by giving people access to education, birth control, etc has resulted in less people having children, as you already said.
While I don't know why that is, I'd guess that a clash between traditional patriarchical values and more modern egalitarian values could be at play. Creates a dissonance that leads to long term relationships failing or not even being considered as an option in these societies.
I don't know in general, but especially when moving from rural to more industrial/city based work, the incentive to reproduce is different. Traditionally, having children has been seen as beneficial in rural economies because the family is building its labor force, and the family works together as a unit to provide more stability overall. Wage labor in cities is more individualized and doesn't require the family unit to achieve a singular goal together, so children begin to be viewed as a drain rather than a benefit.
Exactly, the priorities of women have changed with their entrance into the professional workforce. Society has not kept up with how this changes the incentives regarding children, has not rectified the fact that having children is detrimental to professional careers for women.