You are arguing from an imaginary position. On the basis of the housing crisis, we have convinced the government to spend more than 850 billion to bail things out. You think we're going to do something differently when there are millions of seniors in danger of starving? Under your "plan" all it takes is a single big crash, and then the government will paying for everything for those senior citizens.
We're not going to stand around while millions of seniors starve to death. Not in a Democracy where the largest voting bloc is seniors. Like I said, you're arguing from an imaginary position, like a Libertarian. The world would have to radically alter its nature for anyone to ever take that plan seriously. Even the Republican plan, which was laughable, only called for investment of a third of the funds, and didn't stop you fro paying in.
Social Security is here to stay. It's more likely to simply become a government pension plan, perpetually endowed, than it is to go away. The longer life spans get, the larger the senior voting block becomes, the less likely anyone is going to do anything about it.