Normally you need economic growth to pay off the country's foreign debt. As the now deceased Bavarian minister president Strauss once said, "growth is not everything but without growth there is nothing".
Children are the next generation to pay the pensions of the old, they will be the future service providers, fire brigade, police, doctors, nurses etc. Fewer people doesn't mean you need fewer services, fewer plumbers, fewer electricians, fewer road workers but it does mean there are less taxes to pay for all this. Less consumption means lower production numbers which in turn will cause prices to rise. An infrastructure made for 80 million people will fall apart if the numbers drop to 40 million. Old vulnerable people who have no children to help them will need more support from the state but where are the nurses and money to come from.....Sub-Saharan people, they are the only ones with a population growth.....I can just imagine the nashing of gums from the oldies then.