Under population is not very good and that is what will make a country open their borders for foreigners out of need for labour force. I think there is a solution to this through government policy by giving rewards to couples who increase the numbers of children they give birth to depending on the number stipulated by government. Another point is to discourage abortion because people do that as final resort when they have no other option.
A lot of people do not want kids not because they don't like kids, but because they feel they're not financially ready to have kids and they are not wrong.
Having kids when you can't take care of them is not a good thing to do, so instead of the Government rewarding families that give birth to children, they should find ways to create opportunities for the people, especially young people.
I believe underpopulation is not as big an issue as they make it seem. Don't get me wrong, I know it's an issue but how many countries can show proof that the economy is slow because of underpopulation? Worst case scenario they open their borders for skilled immigrants.