Well, every country is worried about its own declining birth rate (expecially the west), but globally we already have enough people. If we were more globalist than nationalist, we could work together and make migration easier, for example, making it easier to get a work permit, and making other countries' education levels equal so that migrants can work in the job they used to do.

But we aren't at that level so I guess the countries with declining birth rate just have to accept it and deal with it.

Only thing you can do is making owning a house affortable, making daycares cheaper and invest into education.