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).