Political support for Social Security has always been based on the proposition that it is universal and that the benefit reflects contribution. While not required by law -- Social Security taxes are an ordinary income tax per the US Supreme Court -- this has been the explicit contract the government has made and reaffirmed repeatedly with the people for generations. It is an embedded part of the institution of Social Security.

Fundamentally changing the rules at this point would be viewed as a serious violation of trust between the people and the government, at a time when that trust is already at historic lows. Pragmatic politicians also understand that this would greatly erode support for Social Security taxes, since willingness to pay that 12.4% income surtax is strongly connected to the conception that this money will be returned to them in old age. Most politicians have the survival instinct to not even suggest this is not the case, legal technicalities aside.

Honestly we should allow people to opt out entirely if they can demonstrate that they are adequately funding their own retirement/disability on their own plan.

Social Security as it exists now is a Ponzi scheme.