> entire idea of a truly guaranteed pension is not feasible and always leads to a pyramid scheme and then insolvency
Sure. But the point of full faith and credit is it's irreversible. (Actually, the point of credit, period. You can't declare your past debts pyramid schemes and unilaterally absolve yourself of them.)
Irreversibly, like all political concepts, is a fiction that humans choose to believe in rather than a law of physics. If social security became insolvent while the US was distracted by some other future national crisis, I'm certain that the people of the future would find a way to cut the pension while staying within the letter of the law (probably via inflation or https://www.youtube.com/watch?v=TRgRz3nSG7o).
Sure. But the point of full faith and credit is it's irreversible. (Actually, the point of credit, period. You can't declare your past debts pyramid schemes and unilaterally absolve yourself of them.)