There is much dislike for "nationalism" on the progressive-left because -- due to World War II and the Nazis -- they generally equate it with fascism.
But what we call nationalism is the basis of the modern international system usually attributed by historians to the Treaty of Westphalia in 1648.
So, how would a world without nation-states -- which is to say, without nationalism -- function? I can think of two possibilities, although surely there must be others.
One would be an international series of anarchist groupings, small independent townships and farming communities. Sort of like a world made up of an endless series of kibbutzim. The likelihood of any such thing emerging is virtually zero and hardly something that would benefit humanity.
The other alternative is considerably worse. It would mean a one-world government, presumably executed by the United Nations. In other words, the result would be global authoritarianism.
Those of you calling for open borders are calling for precisely that because nations do not exist with open borders. Nations are defined geographically precisely by borders.
If you can think of any other alternatives beyond the nation-state, I would love to hear.