You are right, the global fertility rate is dropping.
At 2.4 currently, it is well above replacement rate but by 21XY we will peak.
At least this is what "technical analysis" of the chart below says.
But can you model the future? The fertility rate has peaks, bumps and valleys in
individual countries and globally. Look at 1950-1965. What if a more renewable,
cleaner, less warming world, convinces women to have more babies?
For your rhetorical question the answer is in the last 2 charts. Energy consumption
per capita, World and US. Over the last 50 yrs, while the US was tightening its energy
belt, the world was having a 50% growth. But notice also, what Volcker did to per
capita energy consumption in the US.