I would argue the concepts of citizenship and passports, along with their Westphalian corollaries of national sovereignty and diplomacy, are a very modern idea

I would argue the concepts of citizenship and passports, along with their Westphalian corollaries of national sovereignty and diplomacy, are a very modern idea that, for the most part, protects people inside of nations from violence, and gives them the stability they need to live and thrive. The whole notion of 'freedom' requires a degree of police protection which doesn't exist, for the time being, without clearly defined national boundaries. The hypothetical you describe, of free-moving people and nations, was the reality for most of human history. This was the era of marauding bands, violent nomadic tribes which pillaged, burned, murdered and raped, not to mention famine and other constant disruptions to daily life. On your way to the French forest you would be murdered by a highwayman. It would certainly be ideal if all nations of the world were available to choose from for all people in a sort of geopolitical marketplace, but that would of course require all nations of the world to have identical values which allowed for the sort of cultural and racial mixing which would surely follow, and I think you'd struggle to find any country that's 100% OK with that.