Most countries are not asking about nationality at their borders because they care about nationality, but because they care about other factors which correlate with nationality and are much harder to measure.
Sure, if you have a completely myopic understanding of the practical realities of immigration.
It's impossible for a country to be sovereign and at the same time not have control over who enters or leaves a country.
So yeah, if you want to get rid of the very idea of countries, it could work...maybe.
So most of Europe isn’t sovereign then?
Can anyone in the world just walk into the EU? No.
That is asserted often but I doubt the truth. Argentinia constitutionally considers migration a human right. They are a minority and not the most prosperous example of a country but they certainly aren't exceptionally worse off even if they wound up with the awkward combination of an overrepresentation of both infamously Nazi refugees and lesser known refugees fleeing from the Nazis historically.
If I was from India, I'd need a visa to travel to Argentina. Not a great example.