we have the evidence of history to show that only national health care works
Yeah, this is related to one of the most tired arguments that gets put forth: the argument that if we change Policy X then "terrible consequence Y" will happen, despite lots of places already having implemented Policy X and Consequence Y not having happened in any of them. There are lots of examples, but a couple relevant to America are the idea that National Health will effectively mean Death Squads deciding who lives and dies, and the idea that reducing gun deaths will automatically lead to an equal increase in knife deaths. We
have 200 different nations on Earth each with different laws, so each nation is effectively a natural experiment in the general effect of different law options. For example, some people might claim that decriminalizng pot will lead to crazed gangs of youths murdering people with chainsaws. Which is all well and good to speculate on, except for the fact that a lot of places have already decriminalized pot without any such effects occurring.
Most fear-mongering about change is in a similar boat to this. Easily debunked by just pointing at places where they didn't happen. A topical example is fear-mongering about all the bad things that will happen if they bring in UBI (everyone suddenly quitting their jobs). Well, there are nations with universal welfare benefits, equal to around the USA minimum wage for a 40-hour week. And those places aren't in fact plagued by pandemic laziness. Only a small percentage of people are happy to live on just that.