Leftoids love to triumphantly brag about how urbanized the American population has become, and at the supposed demographic destiny that will ripple throughout the political world. Sorry, non-urbanites, you've been canceled. Lotsa luck staying alive.
Right-wingers love to parrot the same narrative, only with the emotional value reversed -- it's a sick, cruel joke of the universe that our societies keep getting more and more dominated by urban centers. Cities never sleep from hunting down a nearly extinct species -- the non-urbanite.
In reality, cities have been around for literally thousands of years, and they have never reproduced their own populations. They are endogenous population sinks, as high population density leads to greater epidemic disease burden, higher crime rates, and lower fertility rates. The material standard of living is far lower on average, although with far greater variance -- a few super-wealthy people who cannot be found in rural areas, but teeming hordes of quasi-slaves, who are worse off than their rural counterparts who toil in fields (disease, crime, etc.).
Cities only reproduce and grow their populations from exogenous sources -- migration from country to city, internal to the nation, or immigration of foreigners.
If those external sources dry up, or if there is higher migration out of cities for any reason, then the demographic destiny turns in the other direction -- a supposed future of depopulated cities, ever to be ruled over by country bumpkins and suburbanites. The emptying out of cities has already happened once in recent history -- during the rising-crime period of 1960 to 1990 -- and it will happen again during the next crime wave. Today's return to cities is in no small part due to falling crime rates from around 1990. But crime rates go in cycles, not in one direction.
And the same is true for other forces that affect the degree of urbanization, such as status-striving (pro-city) vs. restrained ambition (anti-city). They move one way for decades, then the opposite way for decades.
These various negative feedback loops keep civilization from heading off in one direction only toward further and further extremes. At most, there is logistic type growth that saturates at a certain level, without going further, such as urbanization resulting from an agricultural economy -- it got more and more urbanized after we adopted farming, but only to a certain point.
In fact, the more common dynamic is an oscillating one -- decades or centuries in one direction, then a reversal for decades or centuries. That prevents a triumphal narrative for either the country or city cheerleaders. The only constant over the long term is tension between the two, not steady erosion of one by the other.
See also: long-term tension between sedentary crop farmers and nomadic livestock herders, where either one could be in the ascendant position for decades and even centuries, only to surrender those victories over the following decades and centuries.
The same is true for the supposed demographic destiny of multi-racial migration. It's possible that diversity will beget diversity, up to a certain point anyway -- that happened when the Indo-Europeans invaded the dark-skinned natives of the Indian subcontinent. And it's apparently happening now in Brazil ("apparently" because of the shorter time scale there, compared to thousands of years in India).
Notice, though, who has maintained dominant status ever since the beginning of those migrations -- not the darker-skinned and tropical-adapted groups, but the lighter-skinned and temperate-adapted groups. So, be careful about wishing for demographic destiny involving racial or ethnic diversity.
But it's also possible that diversity will show oscillatory behavior, whether like an ongoing pendulum or a one-time rise-and-fall. It doesn't look like whites will be returning to Zimbabwe or South Africa anytime soon, after being driven out in the post-Apartheid era.
The Germanic migrations of the medieval period left minimal genetic or cultural traces outside of their original homeland, including where they had been the rulers of the post-Roman period (Spain, Italy). The longstanding genetic and cultural roots of those places proved resistant to even large-scale and centuries-long migrations of foreigners.
Nor did the North African diversity within Moorish-occupied Iberia continue on and on forever -- they got expelled back to where they came from.
And of course the diversity that arrived in North America with the Europeans did not sustain itself -- one group largely wiped out the other, eliminating the temporary diversity of the initial colonial period. Before the Europeans, the Athabaskan wave of Native American migration wiped out the much earlier Amerindian wave in large swaths of Western North America.
This quick overview has left aside the adaptive nature of the political coalition system, which adds yet another source of oscillation to the dynamics. In one period, rural and urban may team up against upstart suburbanites (like the New Deal). In another, rural may side with suburbanites to team up against out-of-control mega-cities (like the Reagan era).
A follow-up post will look at what would happen, just within the left or liberal side of the system, if urbanites determined the future. The black-humor punchline is that the Democrats would only ever nominate Hillary Clinton, and never Bernie Sanders. But stay tuned for a more in-depth discussion.