How large should one state be? The answer differs from a few hundred thousand people to billions.
In the past, unification brought many benefits. The nation become stronger and could conquer others. Its citizens could trade more freely and were safer.
Big states also create problems: They mismanage lands, persecute minorities, and hinder innovation.
I think states should be small (not only in what they are doing, but also geographically). Innovation needs competition. China and Japan produced significantly less after unification. On the other side, Greek city-states were creative hubs. But the article does not describe the superiority of small countries.
The article describes the disappearing reasons for unification. What are the reasons for big states? I mentioned the following:
In some cases, some reasons are dated, in Europe all of them.
Let's imagine some newly created country that secedes from a EU member. I call this country Moravia (we get to more realistic examples later). Since Moravia gets independence by the mutual agreement, it joins all organizations as Czechia did.
Moravia joins NATO. It would command a small but well-trained army. If Moravia fights, it is far from the country itself (unless some world power sweeps Europe quickly). Moravia's citizens would not feel threatened more than if they stayed in Czechia.
It is a bit outdated reason. We don't conquer neighbors anymore.
There might be one reason falling under the umbrella of strength.
A country with a large market can force others to drop tariffs and favor its companies.
"If you let me export, you can import to us."
I don't know how common this is, but the EU coordinates that centrally.
Moravia is a part of the EU. It gives Moravia free trade with its members and defines trade with third countries.
I have no idea how people are proud of being Czechs. They learn in school how great Czechia is, but some persuasion can change their opinion.
People don't think to themselves:
"I don't like the ruling party. Let's try independence!"
So the inertia might be more significant than any other reason.
There are more realistic separatistic groups, namely Catalonia and Scotland.
Catalonia is a wealthy region, so the reason is clear from the side of Spain. State functions in a simple way: the rich are supporting the poor. It works well when the wealth is equally distributed, or rich in cities have strong ties to the countryside.
I'm a fan of Catalonia's independence. They have different culture.
Scotland is not so wealthy, but around Scotland, there is oil. Now, Scotland balances between deficit and surplus based on the price of oil. If Brexit does not work out well (and it can be influenced by the press), then the independence gets more steam.
For Britain, it's reasonable to force the resource-rich part inside, but I see another one. Great Britain was a big empire the lineage of prime ministers and politicians contains a stream of big names. Nobody wants to join the list on the loser side. So it's pride, not the of people, but pride of politicians.
Now, I think the inertia is quite strong. Regions pursue more autonomy than independence. The sentiment might change after some big event.
The trigger might be a change of regime. War has a unifying effect.
Splitting also requires a lot of work. Benefits would appear after a lifetime, so even if splitting is beneficial, it might not be the best investment.
The United States has a different culture than Europe. First, the states themselves function on some level as independent countries. They have autonomy but are interconnected. Second, the Americans are proud of their country being the biggest and strongest. It might make them stay together.
Also, the benefits from the largest army are undeniable.
The absence of military alliances and strong economic zones makes the risks of breaking away higher. These countries would not break off just for prosperity.
Multiethnic countries would benefit from splitting, mainly if there are militant separatists. It is better to look in the future than to have more land with unsatisfied people.
I still don't understand why Africa maintains borders drawn by colonialists.
Of course, I'm omitting a lot of reasons. In the region of minority might be a minority of majority and the majority wants to protect them. Or some parts might have natural resources.
The peaceful country splitting is now a reasonable solution for internal problems. It is more probable in developed parts of the world. The splitting itself happens after some significant change, not by itself.