Social Security was NEVER sustainable. The sustainability of the program when it was presented was an illusion. Its sustainability was based on the US population increasing WITHOUT END. This is a complete physical impossibility, not to mention contrary to all the recorded history of advancing civilizations. It is not now, nor was it ever, a perpetually sustainable program, BECAUSE it is structured as a Ponzi. As long as there is no capital investment and growth, it can not survive, PERIOD. It is simple math (though with VERY big numbers).
A secondary argument against the program is that it is a VERY bad program (just how bad it is becomes hidden behind those same really big numbers). For every person you can point to and say "see it helped this person" or "It kept that person out of poverty" I can point to that same person and run the numbers for them and show you how much money they COULD HAVE HAD instead with a real investment plan. It is a very basic time/value of money calculation.
With the exception of a few first generation SS retiree recipients everyone else for the last 65 years has been hosed.