NO PONZI scheme in the history of the world has ever lasted 75 years. Ponzi schemes depend on garnering an ever-increasing pool of new investors to pay out returns to prior investors. When the potential pool of new investors runs dry, they collapse. This will occur when the scheme runs up against the natural limits of its recruitment strategy; in the ultimate case, it can't keep going past the point where the entire population is already subscribed.
This should provide us with a hint as to why, as Kevin Drum writes (rebutting Shikha Dalmia), Social Security is not a Ponzi scheme. The entire population of working Americans has already been subscribed to Social Security for decades, yet the system continues to pay out benefits on time. That is because the actuarial calculations underlying its revenues and benefits are sound.
Social Security: A monstrous truth | The Economist
AUSTAN GOOLSBEE: I think the world vests too much power, certainly in the president, probably in Washington in general for its influence on the economy, because most all of the economy has nothing to do with the government.