The money was always spent. There has never been a pool of money other than bookkeeping 3rd card Monty.
From day one the money came in and was spent immediately.. The so called surplus was “invested” in gov ious. What does the gov do with proceeds from ious? The spend it.
Ignore all of the political sleight of hand and look at the actual law and follow the money.
My point was that, putting the SS funds into gov’t bonds was a prudent thing to do when the alternative is to leave the cash idle and earn no interest. Later, they didn’t even use the slight of hand of buying Treasuries...they just put the funds into the revenue pool.