Do Falling Birth Rates Pose a Threat to Human Welfare?

Do falling birth rates pose a threat to human welfare?

What does the term “carrying capacity” mean? Is it possible to have too many people on Earth? What does the population have to do with sustainability? What are the key features of “quality of life”? Does the quality of life seem likely to suffer more with a declining population or a growing population? Why is talking about the population problem taboo? Any way around that to have a more open discussion?