Not now, but between 1947 and 1961 there absolutely was. That’s the point: it didn’t happen from the top-down at the Federal level, it happened province-by-province and there was a period of time when it was patchwork. It was fine for the Canadians, and it can be fine for Americans as well.

Also, if I’m understanding your argument correctly, it’s that States can’t do this because borders are open, but there’s nothing stopping a State from applying its public insurance only for established residents of the State. This is how State university systems operate; you don’t get in-state tuition at UC Berkeley unless you can establish residence. In theory, someone at the borders could move across State lines in order to do that, but that’s probably a rounding error in the UC budget.