TL;DR Bondholders can go to zero. Special notes will not.
> The government bond traded for SS and Medicare funds is not sold on the open market.
https://www.stlouisfed.org/publications/regional-economist/j...
"Government debt of the United States is typically issued in the form of U.S. Treasury securities. These securities—simply called Treasuries—are widely regarded to be the safest investments because they lack significant default risk. Therefore, it is no surprise that investors turn to U.S. Treasuries during times of increased uncertainty as a safe haven for their investments. This happened once again during the recent financial crisis. In fact, the increase in the demand for Treasuries was sufficiently large so that prices actually rose with an increase in the supply of government securities."
http://www.cbpp.org/research/social-security/policy-basics-u...
"The Social Security trust funds are invested entirely in U.S. Treasury securities. Like the Treasury bills, notes, and bonds purchased by private investors around the world, the Treasury securities that the trust funds hold are backed by the full faith and credit of the U.S. government. The U.S. government has never defaulted on its obligations, and investors consider U.S. government securities to be one of the world’s safest investments."
https://www.ssa.gov/oact/progdata/fundFAQ.html
"By law, income to the trust funds must be invested, on a daily basis, in securities guaranteed as to both principal and interest by the Federal government. All securities held by the trust funds are "special issues" of the United States Treasury. Such securities are available only to the trust funds.
In the past, the trust funds have held marketable Treasury securities, which are available to the general public. Unlike marketable securities, special issues can be redeemed at any time at face value. Marketable securities are subject to the forces of the open market and may suffer a loss, or enjoy a gain, if sold before maturity. Investment in special issues gives the trust funds the same flexibility as holding cash."
Special issues are literally more valuable than gold, backed by the taxing authority of the US government.