People are probably ok with trade and flow of people, but probably a lot less so if you propose reassigning Alsace-Lorraine to Germany or Provence-Alpes to the control of Italy. In other words the “land” which is what borders are about and protect.

Let’s say there are two countries, Denmark and Sweden. If one day Denmark is going downhill and 50% of the pop would want to move to Sweden because life is much better there and Denmark is going nowhere but down, why shouldn’t it be in Sweden’s interest to one, either stop the flow or alternatively take control and annex Denmark in order to bring it into the fold?

In any case, I see it eventually resulting in a new colonialism where elites and businesses are the main ones deriving advantage and wealth from this new arrangement. Localities lose power to distant and disconnected power.