. . . which means that if the Social Security administrator runs out of $$ and tries to cash one of the “safe government bonds," the money to redeem those bonds has to come directly from the U.S. Treasury - which would then have to borrow the money — exactly as if the “trust fund” did not exist at all.
You are writing about this in future tense, but it's been happening for the past few years, thanks to a wave of early retirements in the Democrat's Great Recession. But, something to consider: where else would you have had the SSA invest it? Until recently, US Treasury Bonds were considered the safest investment in the world. Can you imagine the political football game that would have occurred if they had invested in private assets?
As Vaduz has suggested, the only way to reform Social Security is to switch to an asset-based system. But, it has to be individually-directed, not government-directed. If someone wants to invest in just Treasury Bonds, that's their choice. For people that want better returns but don't want to actively manage it, a lifecycle fund (that gradually becomes more conservative as you approach retirement) would be appropriate.