It’s easier to sell Social Security to people as a “retirement program” than as a pay-as-you-go system because it is just easier to explain it to them that way. Paygo schemes sound weird at first and are easily misunderstood as the number of people who call Social Security a Ponzi scheme clearly demonstrate.
Not sure how I feel about the trust fund scheme. I was about ten at the time, so I didn’t have much say in setting it up. If the money had been invested wisely it would have been nice, instead it was just used to offset tax cuts for the rich. Now the bills are coming due and the rich want to keep their tax cuts.
But the trust fund is there in the form of government issued bonds that have all the weight and measure of US law backing them.