

What is your prediction for what will become of it, though. GDP growth stops and people start bursting into flames? You know we’ve actually observed this before, right?
Now, if you do mean “capitalism” not in the plain definition of “an economy based around private ownership”, but the more specific version where control of capital is highly centralized - there’s some truth to the idea that economic decline can cause people to start looking to reform that system. True of any system, really, because people generally don’t want to see their quality of life decrease. But that’s very different than an economic system “requiring” it to function.
Social Security’s “trust fund” is an empty pit of debt obligations. Benefits to current recipients are paid with incoming payroll taxes. Any difference is made up with additional taxes or monetary inflation, by way of Treasury bonds. They “reinvest it in the economy” if there’s ever a surplus, e.g., through military contractors. It finances the national debt.
Putting aside the quirks of that setup, the basic function is that the taxes you pay in now are not an investment in your own future, you’re basically just paying for the retirement of older people now. The expectation is that someone down the road will then pay taxes to finance your retirement. Hence, how SS was able to start paying benefits almost immediately (3 years) after payroll taxes started being collected.