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.
Everyone who wants to have children is of course truly ready to care for and educate them well and if they are not yet able to care for them and educate them well then it would be better to postpone their desire to have children and continue to improve their financial situation so that they can have children and be able to educate him well. I think having a large population and not being able to produce anything will of course be an opportunity for skilled immigrants to be able to work in that area, so there needs to be an effort from the government to train people to be skilled at work, of course this will create economic growth in that area.