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.
I believe you will also find the number of people too young, old and/or disabled to work has almost invariably been only a tiny fraction of those who are not.
I believe you might want to do a little actual research before spouting off, you are WRONG.
Birth rates in every industrialized nation throughout history have declined (probably has a little something to do with the reduced farming population needed, but that is an assumption). This is a historical fact and easily known if you bothered to look. The ratio of workers to SS recipients began at 7:1 and has steadily fallen ever since (you must ALWAYS keep in mind that recipients are paid with current contributions, not invested capital). From the moment it began, its only destiny was failure.
I get that in your liberal political ideology Social Security is a wonderful concept. I get that. OK, fine, whatever, but facts are facts. The Social Security model that has been saddled upon the American People was crafted by morons, it was not, and could not, ever actually work indefinitely; and the people who put it together knew it, and LIED to gain a political advantage.