It was set up as neither.

It's a taxpayer-funded insurance policy, where the risk is you being alive without a way to earn a living. We have effectively managed that risk for a very large portion of the people who are collecting on that insurance policy by switching over to a service economy where most jobs do not require the physical labor that an elderly person cannot safely do. Furthermore, they're also the portion of the population with the longest time to have accumulated assets.

If you want to keep the insurance policy sustainable for the next generation - who is also paying into it - you need to treat it like an insurance policy. If you allow people to make any and all claims against the funds backing an insurance policy, the funds will eventually be depleted as there will be more people withdrawing from it than paying in. You also have to account for the opportunity cost of the people paying into that system not paying into something else, like our ballooning national debt, or the debts that many Americans of working age have to take on to maintain a decent standard of living.