The population reservoir must not be built within the country itself; generally speaking, population reservoirs are cities and villages.
Take for instance cities continuously absorbing young and middle-aged labor from villages, thereby obtaining a steady stream of population for low-end industries, driving continuous urban development and prosperity, because in general, the natural growth rate of a city's own population is low, and once development reaches a certain stage, there is a high probability of negative growth without introducing external population.
Aging population and degradation of low-end industries are lethal to urban development, as they are to the country. Of course, low-end industries are dynamically developing, after all, technology advances over time, making original industries become low-end industries.
