Except for the doctors that can't afford to treat Medicare patients due to the criminally low/skewed reimbursement rates for some services. There's a reason so many private practices don't take Medicare patients...
Quite the opposite - if the entire country switched to Medicare overnight, Medicare would have to dramatically increase its reimbursement rates, or else hospitals would literally go out of business, and doctors would close up shop. The only reason most can afford to stay in business currently at all is that they can make up the money the lose on Medicare by treating patients on private insurance.
This would result in extreme tax increases - not the proportional amount that you'd expect to cover the remainder of the population, but enough to make up for the difference that's currently being subsidized by private insurance premiums.