Second issue - a major factor driving up the costs of medical treatment in the USA is the regulatory overhead. A medical practice has to hire multiple people just to fill in the forms. If the regulatory burden was removed, Medicare could be viable. But there is zero chance of that happening.
So no - putting everyone on Medicare would just mean everyone would be paying large out of their own pockets while waiting for eventual reimbursement from Medicare - which would only cover part of their costs.
Finally, requiring medical practices to accept Medicare directly would simply mean the medical practices would simply drop back to a 1960's level of care - otherwise they would operate at a loss and go out of business.
Fix issue #2 and the problem would be resolved - and that is not going to happen.