This is trivially fixed by paying people to have and raise children (edit: pay them more than the cost of the children). That no government does this implies to me that it's not really a huge emergency. That this idea doesn't even enter the public forum implies to me that people are still more terrified of maybe-eugenics than they are of falling birth rates, so again it's not that pressing.

Edit NB: paying people more than the cost of the children would cause a lot of poor or dumb people to have kids just for money, so you'd ideally have some standard to meet before you get this money, which is where the eugenics comment applies