At any politically plausible margin, it makes more sense to take $1 out of Medicare than to take it out of Social Security. Social Security checks can be used to buy health care services.
I think that this analysis neglects one key point. Medical care in the United States (or anywhere, for that matter), is hard to bargain with at the time of a procedure (especially an emergent one). It is hard to discuss prices at the ER door during a myocardial infarct, where minutes matter. Here, the real benefit of medicare is the ability to exert market power to set standardized prices (to avoid the power asymmetry otherwise present in medical bargaining).
It is not ideal, but I have not yet come up with a better idea than collective price setting and my own experiences as an uninsured person in the US were certainly eye-opening in this regard. It was amazing how few doctors would even consider accepting cash for services.