True, population growth acts as a tailwind to economic growth. But in the long-run, a smaller population globally is required to avoid hitting the hard ceiling of resource depletion. Population growth only acts as a tailwind to economic growth when other inputs to production and final demand can also be increased. Otherwise, population growth simply leads to shrinking per-capita wealth.
As a net importer of energy, Japan may actually benefit from a smaller population in the long-run.
Japan – Past the Point of No Return – By Vitaliy Katsenelson

