thats the option that I have always preferred, provided the opt in Medicare/medicaid option is provided at cost to the general public.
If set up this way, I dont see why anyone would reject it. At worst, it would be another option amongst many.
The poison pill would be if opt-in medicare was funded from taxes on the general public who were paying concurrently for their private insurance. Medicaid funding would continue to support the poor.
However, this would only be a start. The real problem with US healthcare is a lack of spending controls. European socialized healthcare refuses to purchase treatment if the cost/Quality of life improvement is too low.