Population decline is disastrous for sprawling developing nations like the United states. It’s bad for fully developed nations as well, but it’s cyanide for developing countries. Infrastructure build out requires a strongly growing population, or it’s just not fundable.
For low density countries like the USA, infrastructure is already a disproportionate burden. Population decline will spark a crisis of crumbling regressions and the loss of economic vitality.
The economics of infrastructure actually improve for stable or shrinking populations because funding can shift from raw capacity buildout (extremely expensive) to maintenance and value-add services (profitable). Observe that the best infrastructure in the world is in nations with slow population growth, like Western Europe, Japan, and more recently the U.S. and China. Whereas nations with very high population growth struggle to deliver capacity for even basic needs like clean water or paved roads.
Slow population growth or even stable population is dramatically different from population decline. Usually slow growth is the ideal. But even a modest decline shifts extreme burdens onto wage earners. OTOH, it’s great for buying up property.
For low density countries like the USA, infrastructure is already a disproportionate burden. Population decline will spark a crisis of crumbling regressions and the loss of economic vitality.