It's a Pyramid Scheme. Always was. Such schemes can work in the short-run, but cannot work long term. Just can't.
The problem is not that it's a pyramid scheme - such schemes require a geometric increse in members to sustain themselves. Social Security benefits grow linearly with the retired population (and population growth is even slowing).
The real problem is the unified budget. LBW passed the unified budget, and use the SS trust fund to fund the Vietnam War. Ever since then the trust fund has been a massive fraud. It's an asset on one side, but since the "assets" are all gov't bonds, it's equially a liability.
Calling it a pyramid scheme fundamentally misunderstands the nature of the system.