Once parents are more assured that their children will survive to adulthood, the need to produce enough offspring to compensate for a high death rate is alleviated. Obviously, human reproduction is influenced by factors more complicated than pure sexual instinct. It is not population, per se, that could pose a problem for humanity, but the political and social institutions that affect human behavior. In this regard, the paternalistic welfare state is a serious problem. Government programs that entitle people to consume without their having to produce turns them into drones and parasites.
Energy conservation is an important survival trait.
Individuals that can obtain more goods for less cost will tend to thrive. The welfare state seduces individuals into behavior patterns that exploit this survival trait, but at the cost of imposing extra burdens on the productive individuals. The more generous the welfare benefits, the larger the number of people that will be drawn into this parasitic mode of existence. At some point, the burden of the parasitic portion of the population may overwhelm the output capacity of the productive portion. Thus, it is the ratio of parasitic to productive individuals that is crucial, not the total size of the population.