A social insurance system, in which one generation pays for the previous generation, begins to look more like a pyramid scheme when demographics shift and benefits aren't adjustable downward.

Current social security taxes go to pay current beneficiaries and any surplus is spent just like any other tax money and the federal government issues IOUs to the SSA. That’s fine as long as thier are enough workers paying taxes to pay beneficiaries. But as the working age population gets smaller and life expectancy gets longer, either the government will have to raise taxes or decrease benefits.

There is one other fix - make immigration easier. But in the current climate, a politician could never suggest that.