I just thought I'd put this out there since the original article didn't seem to give the other side of below replacement fertility levels.

That is a great point. However, that solves the short term problem, but not the long term problem. Unless you can say that there will poor countries that are willing to allow limitless emigration to the US forever and the immigration solves the social/welfare problems instead of exacerbating them. Immigrants are usually poor, how are poor people going to solve welfare problems? Immigration has solved Europes population problems, but certainly not their economic/welfare ones.

So, with fertility below replacement and with social/welfare policies that can only be funded by a young society what is going to happen in Europe, Japan, and the US? Are we going to let the old fend for themselves? Or are we going to force the young with little wealth ( but perhaps high income) support the high wealth ( but perhaps low income ) elderly?