When my parents were born, there were about 2 billion people. By the time I was born, the population had doubled to 4 billion. Right now we are headed to 8 billion, doubling once again. In two generations, we’ve gone from 2 to 8 billion. That’s a lot of people needing food, housing, energy, clean water, etc.
While world population is still growing (mostly due to Africa), many regions are actually falling below the replacement level, having fewer babies than those who pass on. Some countries are freaked out by that. Who will care for the elderly. Social Security is founded on population growth. And countries whose self image is based on what people look like (race), worry that immigration will change who they are. Italy, South Korea and China have all tried to encourage people to have babies, an example of the Tragedy of the Commons, doing what makes sense in a narrow view but whose actions undermine the health of the underlying system.
It’s certainly good for the environment to have population stabilizing. But it may be a rocky time as our societies adjust. However this article asserts that artificial intelligence and robotics may help us. Take a look at the graph of regional fertility rates.