But the money is already gone --- and that's with the USA having an extremely fast growing population --- the population of this country doubled in the last 50 years --- that's since the baby boomers were born. The system can't survive even with a rapid growth of population, it needs to be ended --- but as painlessly as possible. Everyone should know the money isn't there so they can make plans not to retire or retire only if they have enough other money saved. It's healthier to stay active and go on working anyhow.
The money collected/extorted which was claimed to be for this specific purpose is certainly gone.
However, the culprit has other assets, and just like a bankrupt company or person can be forced to sell unrelated assets to pay debts, the government should sell off federal property to satisfy the implicit promises it has made.