Saying, "Social Security beneficiaries receive more than they paid in..." indicates you don't know what you are talking about. And, believe it or not, Social Security does invest money.
And yes, it does salvage the argument. Defense spending (all of it not just DOD) is the largest expense of the government. This is expected when you are the hegemon.
It's simply an enormous, vast economy, and 13% of a vast economy is a huge number, large enough to outspend the next several countries combined. That's all.
You could change the law to axe social security, keep its highly-regressive tax, and instead use that to pay for other stuff—but lots of things are possible with a change in the law, and that’s not possible now without such a change. That’s not something that can happen as an ordinary part of the budgeting process.
You are absolutely wrong in your belief. Defense spending is roughly 50% of discretionary spending.