There's a lot more to the demographic transition than that (especially since Social Security checks alone can't and don't keep most retirees afloat financially).
First, with reduced infant mortality compared to 100 years ago, there's less of a reason to have 10 kids with the hope that 1 or 2 make it adulthood, your one kid will probably make it.
Second, back when a large fraction of the population farmed their own plots of land, unless you were wealthy enough to hire labor, the only option was to breed lots of your own laborers.
Third, as women became skilled and educated, having their first kid while in their teens didn't make sense economically or socially.
To reverse these trends, we'd have to A. roll back pediatric medicine so that survival to adulthood becomes a crapshoot again, B. go back to an agrarian economy, and C. treat women the way Muslims do. None of those are going to happen, and none are desirable.
So our economies will just have to adjust to low or zero population growth - there will be a few decades of difficulty with SS and other retirement funds, but otherwise, this isn't a catastrophe. What would be a catastrophe is trying to reverse the zero growth by importing masses of Third World peoples to compensate.
It's a safety net. We know that we can depend on the government for Social Security. 40% of Americans depend on Social Security for at least 50% of their income.
Without Social Security, we'll depend on our families, therefore we'll need to have children.
This is why the fertility rate has declined everywhere including the West, East Asia and even the Gulf States in the Middle East.