This is really a cashflow question, not a budget issue. The bonds held by the SS fund are, from a flow perspective, no different than any other bond, except they are not public and do not have to compete for funds in an open market auction. Today, when a Treasury bond matures, it is simply rolled over in the fixed income markets.My surely oversimplified understanding of things is that the current payments to SS recipients come from1) current SS taxes on those still working and 2) the "cashing out" of the special bonds that SS received from the Treasury in previous years when SS taxes exceeded outgo. That money was spent by the government, and SS received these bonds in return.
So, when those bonds are redeemed for real money (used to pay SS recipients their checks)--the funds have to come from somewhere, and I would think it would be the US government's "general fund." If that's right, then these bonds may serve an important role in helping to "keep track" of which "pot" is owed by which "pot," but the money (represented by the special bonds) is >already< coming from the US Government. From now until these bonds run out the government will will be borrowing, printing, extracting through taxation, or in some other way getting the money to back the redemption of these bonds so that SS recipients can get their checks. So, as a practical matter, how do things change when the bonds run out? If the line item is already in the budget to pay this money in 2033 before the bonds run out, why will it be fiscally impossible to make the same payment the following year when the bonds are gone? I'm sure legislation would be required, and I'd bet someone's ox gets gored, but it doesn't seem that there's anything fundamentally different about the USG balance sheet when these bonds run out. They are an accounting device between various parts of the government, they don't represent a box of gold that exists independent of the federal balance sheet.
But, I am very far from an expert.
The impact here would be how it affects the total amount of Treasury borrowing in financial markets. The key to that is not the total amount of IOUs in the SS trust fund but their maturity dates, and how much additional debt goes out to auction per month.