The US could also thrive with a slowly declining population, as long as there was capital investment to improve productivity at a rate faster than the rate of decline of the working population.

Another factor is that many of the commonly reported economic metrics are aggregated over the whole population, not per-capita. So more people looks like more growth, even when it really means you’re standing still or regressing in terms of meaningful measures of quality of life. And there’s a well-known aphorism in business circles about what happens when you make a metric into a goal.