Open borders is completely unnatural. Humans have always defended tribal property and most wars have been fought over territory.

Borders cannot disappear, you can only collapse the territory down to individual towns or houses, so that people have to shoot people at the gate. We don't have to do that, anymore, because we can assume that they were vetted at the border.

What the border gave us was a larger territory to transverse without having to stop and pay horrendous tolls or be frisked and questioned at every step. Medieval movement was painfully slow and expensive, for citizens. Even just traveling the Rhein, you got stopped and shaken down at every one of the picturesque castles.

We have so much freedom of movement within our borders because we pushed the borders further and further out, like with Schengen.