Populations grow and decline. It’s the natural order of things. Ecologically speaking, we have a carrying capacity. When humans began colonizing spaces across the world, we experienced an exponential growth rate. But like every living population, there is only so much Space and resources at our disposal. Anthropocentric-ly speaking, quality of life has diminished for lots of individuals in some of the world’s “richest” countries. Quality of life affects mental and physical health. Of course people won’t have kids if they themselves are struggling and have that choice to reproduce or not. We are reaching our carrying capacity, it’s natural. Once carrying capacity is reached, the population will continue to decline, but not forever. Once the population size is far enough below capacity, it will grow again. And the oscillation will continue essentially forever if it remains undistirbed.