Nation-states came into existence following the increased reach of reliable, everyday movement of people, goods, and ideas. Such limits are now disappearing, and not even slowly. This will continue to generate friction, while people's desire for belonging morphs political structures into something that suits the new conditions.
A reality in which the citizens of New York and London share more values than they do with people from the surrounding townships, is already here; one cannot wish it away out of nostalgia, sooner or later reaction gives way to progress.
You put it like this is all inevitable and 'progress', which I think is both not true and dangerous by discarding the opinions of those unhappy with it (which leads to things like Brexit, IMHO)
This is a choice made on behalf of the people and Brexit, the recent election result in the Netherland, and the political situation in many other countries show that the people don't necessarily agree with it.
Forced by whom? Changes are agreed by people, often very smart people who believe in a future where we won't squabble on silly things because someone drew a line in 1840something as far as they could drag their cannons.
In a world where your goods and services are built and sold all over the world at any given time, where everyone talks across continents at every hour of the day, a lot of the old "national" dimensions simply don't matter - or keep us in a state of vassalage towards folks who have already embraced the future.
There is always someone resisting change; you can still travel by horse if you really want to, but people will zip by you in trains and cars. Most of the Brexit-supporting public, for example, have already realized that they voted themselves on a buggy whip, and are busy trying to retrofit a steam engine on it.
I think this desire to belong to a group is a very strong instinct, and it will be the strongest during our teenager years, up until we are 25 or so.
It is a biological imperative, not just a cultural construct.
Now, the size of the groups we are capable to create depends on our technology and culture, of course, but not the desire to belong to one.
Therefore, we have sports fans, anime and sci-fi subcultures, and so on.
The instinct is there, but how that instinct manifests is absolutely a cultural construct determined by circumstances. There is no biological imperative that says I should belong to the group of people born less than 10km from me (or 100, or 1.000, or 100.000).
