It's not a policy problem, it's a political problem. People fear change. Most people don't want to abolish private insurance. It should be a case where if you have everyone on the same provider, then there's no stigma for paying for it, but it's just not going to be a law in the United States with its existing political climate... The "Public Option" version of the plan is by far more popular than forcing people onto medicare.
https://thehill.com/hilltv/what-amer...healthcare-for
http://nymag.com/intelligencer/2019/...nderstood.html
https://www.monmouth.edu/polling-ins...oll_US_082619/
^That last one has only 22% of
democrats in favour of abolishing private insurance.
The public provider should, in theory, be more efficient than its competitors, and should therefore win over more market share over time, provided that lobbyists don't create issues to preserve the private participants. Gradually, you might be able to shift the whole country onto the government plan, and once people are comfortable with it and the "fear of change" bit goes away, you can then implement what Warren and Sanders want to do. But with under 20% of the country in favour, it's just not realistic.
EDIT: Basically, most of the USA wants the Santos Health Plan from the West Wing.