Babies!
Individually they may seem pretty basic, but as a group they are nothing less than the future of the world — and most places are running out. In all but three of the world’s advanced economies, there has been a dearth of births for the past four decades. The result is a hollowing-out effect that has aged many national populations past the point of biological regeneration. Such places not only cannot support their social support networks, they are also running out of capital even as their local markets shrivel.
This cannot easily be fixed. Growing a new consumer requires 20 years. Growing a new investor takes 40. And simply educating someone to high school proficiency is a $165,000 endeavor.
For more on how babies (and young people and mature workers and retirees) are inexorably pushing the world into a new age, and how migration will simply not be up to the task of repairing the damage, see Chapters 6 and 15 of The Accidental Superpower.