The 70% don't understand that Medicare for all would kill off the private insurance industry.
A lot of people like the idea of a single-payer system but they never assume they would be forced to give up their current private insurance.
Tell people that their private insurance would go out of business or become restricted to very high net worth individuals, and then ask them how much they like Medicare for all. The polling numbers would drop like a rock. Everyone likes the idea of Medicare for all when it applies to other people but not to themselves.
Perhaps they do undestand and simply don't care? Why should they? Insurance is a parasitic industry.
That's kind of the point of public healthcare. So that regular people would not be forced to have private insurance.
> private insurance would go out of business
I don't know. If my 30M country with public healthcare can have players offering many different policies of medical insurance, surely your 300M country could work it out too.
objectively not true in countries with public insurance.