Social Security is funded by the people paying in, plus the money in funds like OASI. Yes, OASI is treasuries, so it’s connected, but the reason OASI exists is to cover the changing demographics as baby boomers retire. It’s not designed to be, long-term, the way the Social Security system works. The idea is that you collect income on the boomers greater than payouts while their cohort is in the workforce, and the pay that money back out when they retire.

Yes, there are reasons why this isn’t working out as planned. But the idea that “Social Security is the biggest driver of spending” is not a cogent argument.