alt Hacker NewsBut outside the bible, who says we must be more and above replacement rates? Is such a tragedy for humanity if there are only 4 billion humans in 100 years? 4 happier billions? It doesn't HAVE to go up all the time, and between extinction (zero humans) and overcrowding (X billions) there are many unknown ways to go.
Unfortunately most of society and the economy is built on the foundational assumption of a growing (or at least stable) population. There will be some extremely tough challenges to solve with an actively declining population.