1. The government has never guaranteed that benefits or fees (payroll taxes) will never change. They have changed substantially over time (not surprising incomes have risen radically in the last 80 years, as had life expectancy). There are no legal impediments to the government reducing benefits, changing the formula, or raising payroll taxes. Indeed, if they changed the retirement age relative to life expectany back to what it was 80 years, the new retirement age would probably be close to 80 and the system would be overflowing with money.
2. The Federal government owns the “printing press” (ability to create new money without backing) meaning a debt guaranteed by the full faith and credit of the Federal government is secure in the amount of the debt, albeit no guarantee what the money will be worth.
3. Given the America is a democracy, and the percentage of people over 65 is steadily rising (due to longer life expectancy and the inability of the younger generation to reproduce successfully), politically the response to shortfalls in the social security “trust fund” is likely to be keeping it solvent.