We have plenty of people and the birth rate will take care of itself at some point in time. We could have 10% of the world's population and we would still have plenty of people.

The problem is that the modern ECONOMIC system is really nothing more than a Ponzi scheme that requires on ever increasing pool of new entrants to support the previous generation. That's the only real reason birth rates are any concern. We need those workers paying taxes especially in America and Europe. Thus importing 3rd worlders which seemed like a good idea until we found out the 3rd worlders in too many cases are kind of turds.