Whether it's a good idea or not, Medicare for all is politically DOA. Such a program would mean a drastic reduction in the health insurance industry's size, so insurers won't let it happen. Moreover, voters would not accept a change that would be branded "socialist medicine." Even if Medicare for all is good public policy, current American political realities will not let it happen. For now.
But there may be a more politically palatable way to move toward Medicare for all. One of the most popular provisions of ACA allows adult children to continue their health insurance coverage under a parent's health policy until age 26. By introducing a Medicare option for Americans ages 26-30, those who have not yet or only recently entered the private insurance marketplace might find the option appealing. These Americans would comprise relatively low-risk, low-cost enrollees, so an affordable option could be offered. Such an option might even be popular enough to get it passed into law.
At age 31, these Medicare enrollees could be offered the option to continue on Medicare, but if they drop Medicare and return to it later, they would face an otherwise higher premium. This arrangement would incentivize continuing with Medicare rather than jumping on board after they become ill.
In 15 -20 years, a majority of Americans could be covered by Medicare. At that point, Medicare for all might be politically acceptable to a majority of Americans.
Getting to Medicare for all in this way would also reduce the role of private insurance more gradually rather than instantly dissolving the industry as a source of primary insurance. The private insurance industry would continue to exist just as it does under Medicare, as supplemental insurance, but reliance on private insurance would be significantly reduced. The more gradual expansion of Medicare would also allow for the government's Medicare apparatus to grow more manageably.
I'm not going to discuss the reasons I think Medicare for all would be a better system than the existing system, other than mentioning bargaining power and cost control, which is, perhaps, its greatest advantage.