Nobody wants to be the poster child for population decline. Losing 11% in 10 years is serious. Enough for people to extrapolate that to think they could lose 100% in a century.
Bulgaria’s population shrunk more than 11 percent over the past decade, according to its latest census, as the Eastern European country struggles to stem the tide of young people seeking more-lucrative work abroad amid low birthrates.
…Bulgaria has the lowest per-capita income in the 27-member European Union. But since 2014, Bulgarians have been entitled to work and live anywhere in the bloc, with many leaving to seek better pay and career options.
Washington Post
Not mentioned as much as fertility is emigration. A major factor in the near future will be vying for new citizens / keeping existing. Expect some countries to ban people from leaving.