Medicare is actually a benefit to corporations. Corporations shifted retiree benefits (health care among such benefits) to Social Security, 401(k), and Medicare. Without Medicare worker would demand more money. Besides, it is unseemly for a country as rich as the U.S. to have millions of uninsurable people dying prematurely. Millions more than it currenlty has, that is.
Your statements about unions are without a basis is fact. It is quite obvious that labor, overall, has very little power in the country. Furthemore a majority of non-defense workers for the U.S. government are contractors. We live in an environment of regulatory capture. At the very minimum, the large corporations ought to at least pay to keep the system that they've captured.