I would submit that in modern times, just the opposite is happening. Oh sure....we're pretty messed up, we have lots of bad folks, and deadbeats. But hopefully in a few paragraphs, you'll see what I mean.
Africa and her largely homogenous countries is a pitiful collection of infant mortality, corruption, war, famine, and misery. However, three of the four most robust economies on that continent - Nigeria, South Africa, and sometimes Algeria - have a diverse population.
Europe was generally on its way to becoming a socialist hell hole with homogenous countries before the migrant/terrorist crisis. At the same time, Switzerland's population is about one-quarter foreign nationals and one-quarter descended from immigrants - and this prosperous nation has four official languages!!
South America is barely one click better than Africa. It's largest economy, Brazil, and the country with the largest GDP per capita, Chile, both have a population that is about half white, and half "brown" and black (Brazil) or half Mestizo (Chile).
Asia is a hard one to assess, but it is noteworthy that three of the four countries with the highest GDP per capita - Qatar, Singapore, and Hong Kong - have large percentages of immigrants, foreign nationals, or foreign-born.
Then there is the USA: about two-thirds white, 12% black, 10% Hispanic, 5% Asian, and 3% mixed race...and almost ALL of those whites descended from immigrants. From this nation of mutts, this ragtag group of individuals making up 4-5% of the world's population...we mix our labor with 18% of the world's energy output to produce 22% of the planet's GDP. In turn, we hold about 25% of the world's wealth and attract about 30% of the world's bond and equity investment....ALL with only 16% of the world's carbon footprint.
America remains a God-fearing nation. We still have lots of people who go to church and believe in family values. And we remain a nation of individuals...for every Jesse Jackson there is a Clarence Thomas, for every Hillary Clinton there is a Sarah Palin...and for every Richard Nixon there is a Donald Trump.
Yes there is a lot of work and there are a lot of headwinds. But I don't think our fractured gene pool predestines us to a Rome-like fall. We've been kicking butt for a long time, and if God permits it, the odds and data seem to be in our favor.
Though I disagree with this:
for every Jesse Jackson there is a Clarence Thomas, for every Hillary Clinton there is a Sarah Palin...and for every Richard Nixon there is a Donald Trump
Also if I am not mistaken, they are generally segregated into distinct different regions of the country. In 17 cantons German is the official language. In 3 cantons French is the co-official language, and in only one canton is it officially trilingual, though more than 50% speak German.