The money has already been yanked from SS and other programs. It’s included as part of the National Debt figure. They have taken the excess funds for decades and spent it. Having private banks hold and invest these funds instead is highly unlikely.

The government also relies on investment funds to buy treasury bills. Most 401K’s at are set up to give the owner very little control over how they invest their money. All of the offered selections usually include treasury bills. So the government is borrowing money from 401K’s and promising to pay it back as well.

That’s all fine unless something happens to erode the trust and willingness of people around the world to lend money to the U.S. government.