In principle I can see reasons why a community might wish to exclude immigrants from settling down in a particular place. If all the world decided to move to Bermuda, for example, it would be understandable if the Bermudans said 'no'. There are zillions of other much more realistic valid reasons why a community might wish to do similar.
That's a world away from the discussions about immigration to this country, our responsibility for causing instability overseas, our contribution to climate change. We live in a wealthy country which became wealthy (in part) because of Empire. The historical legacy of Empire means we become a target for refugees, people with family, linguistic and other ties to this place. Chickens coming home to roost, and all that.
If all of Bangladesh decided to come here that obviously wouldn't be feasible, but some coming here is fine and contributes to diversity and stuff. But what happens when most of habitable Bangladesh gets flooded by rising sea levels? And so many other nations? How the fuck will the world cope? Billions of people on the move.
Just how nasty could things get, and what would be the consequences?
(sorry if this post is a bit rambly. Huge topic and not enough time to marshal my thoughts properly)