There's nothing unsustainable about that btw. Imagine everyone puts $1 into a central pension fund for 40 working years, until they retire and withdraw $40 at 6

There's nothing unsustainable about that btw. Imagine everyone puts $1 into a central pension fund for 40 working years, until they retire and withdraw $40 at 65. Now just cut out the bank account and have the working people fund the payments directly. Works out to the same given a constant rate of people. There's more nuance of course about the time value of money and demographics, but the direct new->old connection does not imply it's a scheme of any kind. Ponzis require exponential growth in new entrants by contrast rather than just population entry rate ~= exit rate.