In reality, the only way the US General Fund can pay the Social Security Trust Fund is by borrowing more money elsewhere to repay them in cash so they can pay the benefits.
So while in theory what you say is correct, in reality they need to come up with the money to pay. Since they don’t have the revenue they must create more debt to repay the SS debt. Thus no decrease.
I said that the money to redeem the SSTF T-bills must be borrowed, which increases the national debt. However, the national debt is reduced by an equal amount when the SSTF cashes in the T-bills. A wash as far as the national debt is concerned. This is not theory, it is reality.