People disagree about what is damaging to their country. Some people think it's a national emergency that there's no border wall. Otherwise think we live in a third world country because we don't have a single payer healthcare system.

That's why the "evidence" portion is the important qualifier. There is evidence that we pay an absurd amount for healthcare in the US, while there is no evidence that a border wall would do anything.

Others would argue that there's no evidence that the US federal government could improve healthcare outcomes, and that there is evidence that border walls work, say, in Israel.

I'm not interested in arguing any of this either way. Just that "no evidence" can mean "no evidence that I personally find convincing".

No, they wouldn’t — they would just yell the same words louder and keep moving the goalposts so their side could “win”. I could respond with all the differences between the US and Israel borders, or point out that the US government runs two very large single payer health plans already, but that’s not what happens. An actual discussion like that is exactly what democracy needs but seems impossible to have right now.