Regardless, social security is funded totally separately from the rest of the budget, with a dedicated tax, and does not contribute to the deficit. It’s typically excluded from budget discussions, for that reason.
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.
I'm not saying I like everything about federal spending (or even defense spending), just that the argument that we spend most of our money on defense (or even a plurality of it) is false.
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.