Anybody that has a bountiful reproductive strategy in the face of resource abundance should have genes that in about ten or so generations will start to dominate the landscape. Assuming no unusual breaks from history, such as a technological singularity, human population growth should return to a Malthusian arc within ten generations.
According to evolutionists, having fewer kids is a rational strategy for maximizing the amount of parental investment per offspring. For humans, such a strategy is optimal when one's social sphere is highly stratified, such as when there is a prominent nobility class. Why have many kids who get nothing, when you can invest in an alpha or alpha-candidate. In modern times, the alpha strategy means having one super-kid who goes to an elite university, succeeds well after college, and then lands themselves above the class struggle, ensuring the security of their progeny for generations to come.
In modern times, it costs zero, and possibly negative, to have children. Society will give you money and resources just by virtue of having children.