The well-off rich and upper middle-class could move from country to country relatively freely, or immigrants who intended not to look back. Which puts a strong pressure on self-selection on the type of people coming into a country.

That time isn't now, with cheap airfares and the internet, it's much more easier for anyone to come in, often with no intention of integration and bringing their own sectarian politics in. When the time comes, how many of these immigrants do you think will fight for their host country? Especially if said host country make likely come into conflict with their homelands.

Same arguments were just as valid 100-200 years ago where virtually anyone could move anywhere.