...And then, after slaughtering those people and years of insularity, nationalism, and populism... what's going to be left, but for those insular nations to fight over remaining resources?
I can't see a nuclear nation shrugging and saying, "Well I guess we're conquered! Here are the keys..."
Using a nuke to fight off a tide of refugees is like bringing a grenade to a fist fight. Do you think the people who are already here are going to tolerate the wholesale slaughter of their family members who are trying to rejoin them? What about all of their friends? No. If we don't find some way to rescue everybody we will be torn apart by civil war.
If by "here" you mean North America and Western Europe then yes, they will pay the price for their diversity. Places like Japan and China that people say are "xenophobic" might turn out to have the better survival strategy.
Of course not. But do understand that we're talking about large scale warfare here. Even if you're willing to set moral issues aside, it's not going to be cheap. Those who want to isolate their countries from "third world problems" are going to pay for it in cold hard cash. Thus, even they cannot truly claim to be unaffected.
I can't see a nuclear nation shrugging and saying, "Well I guess we're conquered! Here are the keys..."