The complicated aspect about the Social Security system in the United States is that it was falsely marketed.
It's called an "entitlement" because people pay into it and are supposed to get it back like a pension, regardless of whether they are rich or poor when they retire. And so the Baby Boomer generation views any cuts to their social security as a rugpull, basically. It's not insurance or charity; it's an entitlement.
However, although it was marketed as like an entitlement/pension, that's not how the math worked out in practice. And it's because population growth is slowing. It was based on ponzi math, assuming that every generation will be bigger than the one that came before it. But the Baby Boomer generation was huge.
In addition, when Social Security was created, the retirement age was set near the average life expectancy. Many people would not live long enough to collect it, and most would collect it for a handful of years. Only a small minority of outliers would work for like 40 years and then live off social security for like 20+ years. But then over the decades, life expectancy increased by like 15 years, so the default assumption is indeed that someone can work for 40 years and then have 20+ years of retirement, even though the amount they pay into it doesn't really mathematically cover that. It's not designed for that en masse.
And so Baby Boomers had like a 3.5 worker-to-retiree ratio to support in their peak earnings years, while Millennials will have more like a 2.5 worker-to-retiree ratio or less to deal with. Which means they get a worse deal. Many Millennials don't even think they'll get it at all, despite paying into it.
That breaks up the social contract and sets up inter-generational political conflict. "Fourth Turning" stuff.
It's a big reason why "defined benefit" plans are inherently unstable; they rely on being able to predict the future.
And it's also a big reason why, when speaking about deficits, nothing stops this train.
