I am a Canadian and we have the exact system Sanders wants in this US. With very few explicit exceptions, private medical insurance is illegal and the government is the only insurer. It is also not possible to pay for or provide supplementary services, ie, if you have to wait 2 months for something, there are no private clinics that can do it in 2 weeks.
Universal Healthcare means everybody is covered and there's many different way to provide such coverage. Canada's is not even a very common approach.
There are numerous private clinics that can get things done fast, and Canadians often use them. Things would be different if the border were closed or if Canadians lived far from it.
Without that easy-to-cross border, the political pressure to allow private clinics would be irresistible. The law would change.
Universal Healthcare means everybody is covered and there's many different way to provide such coverage. Canada's is not even a very common approach.