There are almost 7.7 billion people on the planet already, even with less than replacement birthrates, I think we're good on human count for the next 300 years or so at a minimum so unless the birth rate really catastrophically collapses. We could lose an order of magnitude of people without endangering genetic diversity, take significant load off of the environment, and frankly, quite probably improve the human condition with fewer billions of other people to compete with in the world. I recommend seeing how we can extend this trend to demographics and countries it hasn't reached yet.