It literally is set up like a Ponzi scheme. When SS was created there were seven workers paying in for every one retiree. Today there are less than two paying in. It was set up with the assumption that everyone would keep having five, six, seven kids, but "the pill" got invented 30 years later. Eventually, the people at the bottom of the pyramid won't be cashing out without major changes like raising the collection age or increasing the tax.
I'm 41. I don't even factor SS into my retirement planning because in my opinion it's a 50/50 chance it will be insolvent by then after your generation bleeds it dry because you refuse to change. Bush had the right idea with the private accounts and he was wrongly demonized over that proposal. It was the only reason I voted for him in 2004.