It's really not that abstract. Phrased another way, would you want the world to go back to a state where the sun never set on the British Empire? Most people wouldn't, including most Brits, even though it was a large free trade zone (the "Imperial Preference"). There are more borders now, but that's offset by the benefits of localism. The great empires were not famous for their love or respect of local cultures.
WW1 was a good example of where a lot of those countries came from. The empires of the day were very large, not many countries, and the people running the German/Austro/Hungarian/Ottoman empires thought that was great and maybe they should reduce the number of borders in the world a bit more. They were defeated and in the aftermath a lot of new countries sprang up, for instance, the modern concept of countries didn't really exist in North Africa/Arabia until the Ottoman Empire collapsed.
And the argument about people from other cultures not always mixing well isn't abstract. That is a lesson being learned in blood. The killer of Paty was an illegal immigrant, that's why Macron is suddenly pro-borders.
The point is, the modern left treats borders as some unspeakable evil, but never make any kind of intellectual argument against them, probably because the moment you do try to grapple with this issue intellectually and the history of where this system came from it becomes complex. Probably also because borders are an aspect of localism and the left historically is a globalist ideology. So instead they just attack the infrastructure and skip the whole thought process entirely. It's anti-intellectual.