This is a list of things that can go away in a heartbeat. Someone living in Victorian England couldn't possibly imagine a world where Britain wasn't at the absolute centre and on in charge of everything, yet within two generations that power had all but faded.
What if Texas splits off and takes a few states with it? What if California decides to bail as well? That'd put a lot of pressure on New England, which might just call it quits, too, rather than be saddled with the South.
A single thing could lead to the complete erosion of the United States as we know it, and politically it would never recover.
If you think this is impossible, that it will never happen, the Austro-Hungarian empire was once like this, geographically and economically dominant, yet it fell apart surprisingly quickly.
Consider: The United States has only fought only a handful of wars on its own soil and a few of those went really badly. A large country is harder to defend, and if the entire might of the US military can't keep a lid on a place like Afghanistan or Iraq what hope does it have holding together half a continent?
There are good reasons for this, one is unity of culture compared to most other countries in the world, and another is federalism to tide over the differences. Things could change if say, cloning started happening and the South couldn't handle it, or Trump became a dictator-for-life and institutional means didn't stop him, but somebody would have to change their way of life for secession to be seriously on the table again. Why would California or Texas break away today? Internal economic dynamics are beneficial enough to keep them in. Things like language differences that might break apart other centralized countries simply aren't issues (some localities print their official stuff in Spanish, Vietnamese, whatever, others don't; ethnic radio stations and TV stations exist if you want to have them, nobody cares; you can incorporate your own cities if there are enough of "you" living together, whatever "you" are, and you can elect one of your own as mayor, it's a big country). During the Civil War there was still a strong notion of states' rights, today there is not and the country is a lot more similar than different.
Also Texas was independent. It chose to join and wasn't forced in. Texas seceding is just a joke. There is a lot to be said for a country built on strong institutional legitimacy (even the re-organized South after the Civil War was re-admitted state by state through local votes): people don't jump for the exits at the first sign of trouble.
California could quit if a dictator arises, a threat that was unthinkable a decade ago but is now unnervingly probable. Other states like Vermont would likely cut loose as well, they don't have time for that.
> During the Civil War there was still a strong notion of states' rights, today there is not and the country is a lot more similar than different.
I'd argue that people more strongly identify with their home state than with some vague concept of "America", especially when it comes to states like California, Texas and Florida that have their own identity. The more these states differentiate, the more likely they are to disagree, and from there, eventually split.
Europe's had a long time to end up in its current state, and it's still shifting wildly. Belgium might split in half. Spain might rip apart. England might lose Scotland. Ireland might get unified. Nothing stays static for long.
Consider the United States in its current form only really came about in 1912. That's basically yesterday in terms of history. To think that will remain static for another hundred years is ridiculous.
> within two generations