The real world is not some dystopian SF ruled by cool kids wearing shiny armor. The world's economy is still run by people consuming goods and services, creating demands - declining population is not an insurmountable problem but it does pose a huge challenge for any nation's economy.

Fewer people -> fewer sales for Ioniq and Galaxy S -> less money made by these companies -> less money available for industrial research -> worse technology for your servers, drones, military satellites, submarines, whatever -> lose the war.