We might (for instance) discuss migration of the mass of newly educated, skilled labour from Africa. It could offset this decline in the developed world. It could also allow capital inflows into Africa, as remittence money does flow into Indonesia, Malaysia, Vietnam and the Philippines.
Merkel was probably right, in macro-economic terms, allowing a huge influx of refugees for future labour needs in Germany. (thats pretty much what the Turkish migration of the post-war years was about, as was the british colonies migration of the same period into clothing, manufacturing and services in the UK)
Some of the work-age labour hole is filled by automation and robotics.