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.
Of course, under population is a problem. A country can face some problems in the future if they have under population. It happens in Japan, they are facing a problem of under population. The number of foreigners is increasing constantly to deal with the situation. If too many foreigners, I think it won't be good for Japan in the future.
Well, this problem is probably caused by the lifestyle, the economic situation, or lack of confidence from mature people to raise children. The government should make a special program to all people that they must be confident to grow children. The government must support with assistance programs, so there are no mature people who doubt to have children. Also, the idea to give rewards for the couples that have children seem good to apply as well.
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https://edition.cnn.com/2023/04/13/asia/japan-population-decline-record-drop-intl-hnk/index.html-
https://www.dw.com/en/japan-can-anything-be-done-to-stop-population-decline/a-66432824#:~:text=Japan%20now%20has%20122.4%20million,and%20tried%20to%20offer%20solutions.