It does function as a Ponzi because it relies primarily on new "investors" to pay earlier investors.
As it becomes more difficult to get enough new investors to pay the earlier investors the inevitable collapse of the scheme occurs.
The surpluses are not "locked" away for the benefit of the investors, they are looted and used for the benefit of the administrators of the scheme, government.
When I retire, I will be one of two or three earlier investors for every new investor.
Some models predict a one to one ratio of earlier investors to new investors.
Based on the ratio of one to one, the government has promised me on my latest SS statement to send me $1,800 out of one other person's monthly income.
Only 2 scenarios are possible.
Either the person sending me 1,800 dollars a month will have to be paid far beyond their productive capacity or the person will become a welcher.
Oddly enough, this is all exactly what I would expect from a program that at its inception set a retirement age beyond average life expectancy.