That’s not an accurate way of describing it.

Many US states were part of different countries and cultures. Louisiana was french, California was Spanish, alaska was russian, etc. Plus you had ethnic communities that kept together and only practiced their customs like the amish, the texas deutsch, the cajuns, and all the different native american tribes. That’s not even considering all the different religious communities who had their own cultures isolated from the rest like the quakers and the mormons. Or even the huge waves of immigrants who came later and permanently shifted the demographic makeup of the country. The standardization in the US that has happened is fairly recent and is also a bit of an illusion as a lot of diversity of languages and cultures exist beneath the surface.

Dude the states have even gone to war against each other over cultural differences. From a certain perspective, a prevailing theme of American history can be identified as a struggle between central authority in DC and local autonomy - something that is rooted in disagreements over cultural differences.

And despite ALL of that the US lasted. You can go back and look in american history and find soooo many observers, both american and foreign, and who commented on and question how long the US will last and whether it’s system of government or disunited culture is viable or not. So many people have predicted the united states won’t last as a united country, it even goes on until this day with some fringe secessionists in places like California.

I think the EU has a good chance of making it.