Japan has a completely different pension system which is extremely conservative.
It's a two tier system, the first government funded, and of course the Japanese government is in enormous debt, but not because of pensions. The government funded pension is like welfare, it's very small, is fixed in amount for everyone, and funded by contributions. It would be what social security is if the amount was tiny -- say $200 per month -- and fixed for everyone regardless of how much they pay in, even though everyone pays in a bit less than 1% of their salary. It is a system not dependent on positive rates of return.
The second system is the main source of pensions for people and is employer/employee funded. It is also quite austere -- you pay in 18% of your income in year 1 and that amount increases by 0.25% each year. Of course only half is deducted from your paycheck, the rest from the employer, but it is effectively taken out of the worker's paycheck.
What you get when you retire is ~0.55% of your total lifetime wages each year. This amount is fixed in nominal terms -- there is no inflation, cost of living adjustment, etc. You get this fixed amount every year. That means that by design the system is solvent because even if nothing is invested and the money earns 0%, if you work for 30 years and never get a raise, you will have paid in an average of ~22% of your lifetime wages which will fund 32 years of retirement pension after 30 years of working. But a normal person will get raises, and as they get raises over time, they earn more later in life when their percentage contribution goes up, so pensions are well funded even for a population with long lifespans and low interest rates. Then of course people have private savings in addition to pensions.
Needless to say, Americans would riot if you told them they need to increase their social security contribution from 15% to 18%, with no cap, and that this amount goes up every year, and then what they get would be only 0.55% of their lifetime earnings with no COLA. That means someone earning the median per-capita income of about 31K for 40 years of work would get $570 per month in social security benefits after paying in an average of $594 per month over 40 years. But again, our system is designed for people who die younger and work in an economy with decent rates of return.
But if we adopted a Japanese style system, social security would be permanently in the black regardless of interest rates.