It is not possible to maintain funds worth six times the value of GDP, not nationally, and particularly not globally. The supply of productive investments would quite simply run out. An economy with real assets worth more than six times the value of GDP would be strongly overcapitalised, and its return on capital insignificant.
Thus, it is clear that a large part of the funds needed to pay our future pensions will inevitably have to be covered through pay-as-you-go schemes.