These things form when the will or the power to implement is there. I would think the possibility of such a unified cross cultural & properly effective institut

These things form when the will or the power to implement is there. I would think the possibility of such a unified cross cultural & properly effective institution would be down to power & strategic interests i.e. game theory. Humans will seek power when it's available and wield it to do things in their interest. Also a world government making decisions on "immigration" whose effects are distributed unevenly on its constituents. Look at Poland in the EU right now. https://www.bbc.com/news/59348337 https://www.bbc.com/news/59348337 "We could replace the war model with the policing model". How? Total fascism or weaker punitive measures? Also does the omission of one correlate strictly with the absence of the other? And over what time scale? Look up "Mediocristan" & "Extremistan" to get an understanding of how the urge to control from a top down level can blow up. "we could institute the basic human right of mobility on the planet" some ecosystems cannot support free mobility.