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.
I don't think it's a bad idea to employ the skills and services of foreigners by countries that those manpower skills and services are lacking due to underpopulation, if a country fears for it's future by opening her boarder to foreigner with the needed skills to troop then the government of such country's would be doing a disservice to her people as they would have to work constant overtime and that can lead to a more breakdown in human capital.
Why can salvage the situation is just as you both agreed on the government to create a programme that will encourage couples and single matured people to increase their number of childbirth, cause where the necessary
social amenities and welfare service are working fine people won't think twice before giving birth knowing fully it won't turn out to be a burden alas.