Most advanced countries are experiencing between flat and declining populations. The steady state fertility rate (births per woman) is 2.1. South Korea: 0.85. U.S: 1.7. The only way to keep population steady or growing is longer life expectancy and immigration. Canada one of extremely few countries with a functional immigration system which imports people (about 1% of total population a year, compared to about 0.3% in the U.S.).
Population decline creates problems: (1) faster shift towards old, post-workforce population, i.e. more old people per worker; (2) constraints on workforce supply. But constraints need to be examined by sector (personal services, technology, scientific, retail, etc.). And they are open to solutions, which may be more available with the penetration of artificial intelligence, with its capabilities in robots as well as human-like cognition. Social and political forces do, of course, constrain the flexibility of our immigration policies. There is no evidence that declining population adversely affects productivity improvements, but maybe I’m missing something.
Other problems from population decline include the risk of deflation. Deflation increases debt burden, discourages investment, and makes it more difficult for a central bank to manage the economy.
Another is that politics and enterprises have less new blood flowing into them, raising the risk of organizational sclerosis.
Population increase can be by increasing fertility rate (very few successful examples, such as Hungary) and immigration. Immigration is a fruitful form of population stability and increase for two reasons: (1) immigration (especially at middle class level creates) a more cosmopolitan, liberal democratic society, which most of us want for social, cultural and economic reasons. (2) immigration tends to concentrate at child-bearing age (20-40) hence a fertility boost. (Immigrants are 14% of US population but 20% of births).
New strategies for immigration can lessen workforce constraints due to aging. Example: A consortium of hospital systems makes a 20-year deal with nursing schools in Nigeria to provide a steady stream of nurses. California Ag companies make deal with a Mexican state for farm labor. These will not happen until immigration control is shared with private organizations. These are basically rough copies of how the AMA influences the immigration of medical doctors.