Maybe. Even today many doctors and private hospitals won't accept new Medicare patients due to the caps in the payment schedule. If you assume Medicare would have to accept anyone who wanted to buy in, irrespective of preexisting conditions, then it would have much more inherent risk. Which means higher costs or even lower payouts, making it even less desirable for health care providers.
Medicare for all would have to be a single payer system with all other forms of health insurance disbanded. Obviously this is the end goal for those who promote it.
*MySmiley*
"Bustin' makes me feel good!"
Ghostbusters, by Ray Parker Jr.