[soco]

But 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.

[tskj]

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.