> So if the program were cancelled, the bonds could revert back to the US government - in other words a bunch of debt would be forgiven

That seems like a passive-voice way of saying "We could steal from millions of people, pocketing the retirement savings they worked for their entire lives."

Also, there's nothing stopping Social Security from upping the cap and thus returning to surplus, aside from the group of people who would absolutely love to steal from the fund.