Earlier quoted context omitted.
Fundamentally any retirement plan that doesn't involve canned food in a celler is pay-go because the retirees will be consuming resources from the economy they retired into rather than the one they were workers in. Their retirement savings can be in public company stocks instead of government guarantees but that only works if there are people willing to buy those stocks (ex: younger people saving for retirement).
That's only true if the economy doesn't grow i.e. productivity doesn't improve. If retirements are funded from returns on assets they scale with productivity. If they're funded by current workers they're bounded by the limits of population and wage growth.
In a world with forever imploding consumer aggregate demand, there is no safe asset.