I question the idea that it was ever an insurance plan, unless the associated risk was living.
The progressive movement took its model from the European state pension systems, which were more or less general treatury items.
That SS ever took in more than it paid out was a bit disengenious, in that Congress has always treated the SS surplus as a money cupboard.
It was always a Ponzi scheme in that it required an ever expanding base of payers to avoid bankruptcy.
Citation: An act to provide for the general welfare by establishing a system of Federal old-age benefits, and by enabling the several States to make more adequate provision for aged persons, blind persons, dependent and crippled children, maternal and child welfare, public health, and the administration of their unemployment compensation laws; to establish a Social Security Board; to raise revenue; and for other purposes, August 14, 1935; Enrolled Acts and Resolutions of Congress, 1789-; General Records of the United States Government; Record Group 11; National Archives