Thanks for the very thorough post, and please don’t be disappointed if I do not respond point by point, as that would be an unreadable post. However, let me raise a few issues:
First, and again, one cannot compare a legally unenforcable IOU to one’s self with a bond backed by the full faith and credit of the United States. The bulk of your post did not deal with this issue, of course, other than your opening statement that you do not concede the point. I’m sure that nearly everyone else understands that there is a fundamental difference between a legally worthless piece of paper and one that is guaranteed to have value under the Constitution, even if some posters here quibble with that obvious distinction.
Second, you say that money is fungible. As a general principle, and in most applications, that is entirely true. But the difference in this case is that the fungibility of money is limited by law. At the very basic level, no matter how much is collected in income tax, none of those funds may be used for Social Security retirement benefits. Similarly, funds appropriated to the Department of Interior may not be used for military salaries, even though both agencies are part of the Federal government.
This is important because the SSTF is, in fact, practice and law, a system designed to limit fungibility. The whole intent is that FICA revenues over time dictate the solvency of the SS retirement system. If money were truly fungible from a government-wide perspective, this would not be the case. But if my mother had four wheels and a transmission, she would be a car.
Finally, all the numbers that you presented about whether the SSTF makes a difference in the overall Federal budget are pretty much on-target. But the point of the SSTF is not to make a difference in the overall budget, it is to insulate the SS system from the budget dynamics of the rest of the government. Just like how corporations often own subsidiaries that have an independent legal identity from the parent (like how Otis Elevators and Sikorsky Helicopters are owned by the same corporation, but function independently of each other) the Social Security system has its own identity in order to achieve various policy objectives. Your arithmetic, while pretty much on the mark, does not address this fact, which is the central issue in this discussion.