Earlier quoted context omitted.
Considering that’s never actually happened it seems you have an unreasonable level of certainty about the outcomes. People who don’t have kids don’t pass down their culture, which seems like an obvious feedback loop to keep things going. On top of this population declines have generally resulted in increased individual prosperity for hopefully obvious reasons.
And you are wrong about individual prosperity being a result of fewer people. Extremely wrong in fact. In a village of 100 people everyone is poor. Part of the reason US is becoming less prosperous in the last 50 years is because of this collapsing birth rate.
What confuses things is local areas tend to lose population due to economic decline and prosperous areas attract people. However, when the decline is on much larger scale things flip.