Social security is not sustainable. For one thing, we don't have a workforce to support social security payments and the work force that we are moving towards is to a majority of low paid, low level service workers who can't possibly hope to support social security.
Republicans want to have a system where people can invest their money into whatever retirement plan they choose. Someday, except for a thin safety net, Social Security will be replaced. For those people who have social security and private investment income, which pays more? If given the option, most people, or intelligent people at least, would choose private investment over government confiscation. For those that prefer the pittance of social security, they will be able to choose the same method already used.