Maybe you should read more about it. If you know how many people are going to be aging into the Social Security system, and needing Medicare/Medicaid, and some simple assumptions about the budget and tax revenue, you can easily generate the approximate shape and amounts of those financial projections.

It is so simple a calculation and so ridiculous a situation, as you say (but in a different way than you mean), that this is a huge problem that will have to be confronted.

The CBO generates those kinds of calculations, and they're not a group known for making up things out of thin air.