The difference is the government can always get the money for the retirement folks, one way or another. Raise taxes or cut other programs, for example.
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That sums it up in a nutshell.
Check a pie chart of gov't spending. SS is going to keep growing in that pie, as life expectancies continue to increase throughout the century. Eventually, it will crowd out many programs that most of us would see as unthinkable to cut today. Theoretically, it will never stop expanding within the pie.
It's a monster, really. Or maybe that's too fear-mongering. But it's insane to think it's sustainable as a gov't program. It was flawed from the start.