Far more likely they will just reform their immigration policies to make it easier for people to move into the country. Increasing birthrates has a long lead t

Far more likely they will just reform their immigration policies to make it easier for people to move into the country. Increasing birthrates has a long lead time, it takes at least a generation from when incentives begin to when you start seeing an increase in the number of economically active tax paying adults. And due to population momentum and cultural inertia, it is likely to take much longer than that still. Immigrants on the other hand can change the shape of a population pyramid on the timescale of a political administration and they start contributing to the economy immediately. Further immigrants tend to come from higher birthrate regions and will likely continue those practices for at least a generation after arrival, leading to long term improvements in population growth. Not every country is a desirable destination for immigrants, and some cultures have stronger barriers than others to assimilation of foreigners, but for many nations growth should easily be possible so long as there are people looking for a higher quality of life, which presumably will still be the case even decades after the world's population levels off.