The real resources available to fund pension benefits depend on the economy's long-term growth rate, which in its simplest terms is determined by the growth rate of labor employed plus the growth rate of the productivity of that labor. As already noted, by 2030 the growth rate of our working-age population is expected to decline by half.
The fraction of the working-age population actually employed will doubtless be affected by improvements in health or changes in the economic returns to working. Labor productivity has historically been affected by changes in the amount of capital available to each worker, the pace of technical progress and, perhaps more subtly, changes in the experience of our workforce.