Why not transform the whole thing to a 401k type set-up for anyone under 57 yrs old?
The stated reason is the benefits other than retirement that SS provides... income for your spouse and minor children in the event of your death, and a disability payment if needed.
The second stated reason is the idea that the government will "assure" these benefits which presumes that the private sector might not.
The third reason, of course, is that they need the money that those people under 57 are putting in now to pay those people who put in for 50 years and are now collecting. I think ideas like Thompson's (and Bush's before that) to change SS recognize that such a change will need to be gradual and across a generation. You can't just take a group and tell them "Nothing you have done matters, we're going to fix this on your back." It simply wouldn't succeed politically or morally for that matter.