In the private sector, if you 'reduce benefits' that basically means you defaulted on your obligations -- i.e., bankruptcy. So, I guess in a sense it can't go bankrupt, but it seems a meaningless tautology at that point.

The government can also print money to cover the expenses. But, those are the only 3 ways to make it continue: reduce benefits, raise taxes, print money. They all have a practical limit. Eventually, a system that can't go on forever, won't.

Critics pointed out it would eventually collapse when it was being debated in 1935. FDR didn't care -- he knew it would be a political winner for generations.