A nation without borders has two opposing faces. On one side it's a politically correct, progressive statement about freedom and equality. On the other side , however, it is a statement about losing national identity, pride, and security. It would seem that the idea of having international borders is no longer compatible with the politically correct ideas of freedom and equality.
Statistics on the global participation rate in both the internet and mobile devices seem to suggest that borders are going out. With an estimated 3.6 billion internet users in 2017, and the statistic that there are now more mobile phone users worldwide than thre are people, it can be said that we are well on our way to becoming a world without borders. Never before, in the history of humanity, has so many people been able to communicate with each other so easily and quickly.
The idea of nationalism would seem to only be feasible in a world with borders. In a homogenous society in which every person is free to come and go, and stay, as they please, would be in direct competition with nationalism, national pride, and national security. Imagine if China were to open its borders to allow its poorest people to move, 170 million of them in 2013, to any other country. Word would quickly spread online and any one of the largest countries in the world with that influx of immigrants would just as quickly have its infrastructure, police, fire and rescue, healthcare, culture, and its very national identity in a crisis. It's aa fact that diversity kills empires.
The fact that modern communications and ease of transportation would make it possible for the world without borders to change overnight, doesn't mean it would. It's far more likely that the politically correct ideals of tolerance and acceptance will gradually force homogeneous populations on any nation that refuses to reinforce its nationalism and national pride. Any such nation would become home to a plethora of competing cultures who's resentment for each other would slowly destroy it. The culture with the higher birth rate would eventually overrule and drive out all others. Agricultural jobs and other hard labor workers would initially be forced into unemployment as new immigrants took such jobs for a fraction of what native workers earn. A generation or two later and the immigrants would flood the white collar work place as well, further driving down wages. The gap between the wealthy and the poor would grow considerably.
Taking pride in one's country and its best history, finest traditions, and its culture are not a bad thing. It doesn't mean looking down on other cultures or immigrants. Being a nationalist doesn't mean that you have to think of your nation's history, culture, and traditions as better than any other either. It simply means taking pride in your heritage, working to protect it, and welcoming immigrants who wish to participate in it. A nation's borders and their accompanying walls or fences shouldn't be intended to keep anyone in or out, but rather to define its lands and encourage immigrants or travelers come in through the front door.