For those who insist that cheaters are mostly anecdotal, you should know that, from one who administers the program, there are a stunningly high number of cheaters, more than you could possibly imagine--I deal with them virtually every day. Indeed, it may interest you to know that there are so many cheaters, our federal enforcement branch won't even prosecute anyone unless their fraud amount has grown to a shocking number because they simply can't worry about "little fish".
A couple facts that may be of interest: First, one of the most significant arguments from those who actually are studying the alternatives to Social Security is that if we went to a strictly voluntary privatized system, there would be many, many, many people who simply would refuse to contribute to their own retirement and spend their money on today (just look at the savings rate in the U.S. to see the proof). This would create an entire class of people--probably a majority--who would come to their retirements and have nothing. Thus, a Social Security system would have to be created for them.... And since they would be a majority, or at least a huge minority, they would have the political power to create such a system for themselves. So we would actually be worse off than we are now. Until that problem gets solved, a privatized system (which I am moderately in favor of) will not be advanced. And as you think about it, you can probably figure out that...the problem is unsolvable.
Second, certainly the most significant threat to the survival of the program is not the cheaters but rather the constant multiplying of entitlements by politicians and bureaucrats in order to curry political favor. Regardless of the amount defrauded by cheaters (and it is huge), it is a drop in the bucket compared to the pandering of politicians as they seek desperately to invent another program which will hit the correct (i.e. the correct voting) demographic...and then the bureaucrats in Washington D.C. go wild (you have no idea...you can't imagine) creating policies for those programs which encourage entitlement and discourage any self-reliance or integrity, in the clients and in the program. You have to remember that, in any bureaucracy, those who are more interested in the survival and the thriving of the bureaucracy always take control as time goes on over those who are interested in the mission of the agency. And for a bureaucrat, the more the program expands, the more money congress pours into it, the richer and fatter he gets, and the more job opportunities and promotions and fame and power are available to him. Sigh....