Does post nationalistic mean no more nations or cultural identities? No. Again, cultural identities have existed for eons and they will continue to exist. That being said, humans started off in families, which grew to tribes, then cities, then states, then united states in the form of nations. The next logical step is post nationalism. Short of some extraterrestrial discovery or violent worldwide event, I expect the transition to take a very long time and likely have quite a few bumps along the way. That being said, a proper understanding of ethics should help this process along; hopefully peacefully.
All humans have the same foundation in ethics. Ethics transcends time and place. It has always existed and it will always exist, in the same form; it is non-transient. The only thing that changes over time is personal and group understanding and application of ethics.
While local cultures may solve conventional matters in different ways, and therefore provide great diversity in the forms and functions of life, the application of ethics should provide consistency among cultures provided their understanding of ethics is consistent with every other cultures. Given the relative starting points, I imagine this should take some time as well.
In your opinion, does post nationalism necessitate economic globalism (free trade)? I am skeptical of globalism but I think my concern is different from most people’s. What I think of when I think of globalism is that rich nations roll into poorer nations with cheap labor/resources, exploit the local people and environment, causing a lot of human and animal suffering. Because this suffering is happening so far away, it’s difficult for most of the people in the rich nations not actively involved in these activities to be fully aware of what’s going on. In some cases they may turn a blind eye to it because it’s easy to ignore suffering that’s far away, and in many other cases they simply don’t know. A lot of electronics are made with cobalt, and most of the big tech companies have been caught buying cobalt from mines that use child slave labour. It’s pretty much certain that one, if not all, of the electronics I own are the product of intense human suffering. I would be willing to pay higher prices and own less things if I could know this was not the case, and I suspect people would be less willing/able to overlook child slavery if it was happening in their own countries. This would also reduce demand for child slavery and improve environmental conditions for people living in the areas that currently have child slave work camps.
Nationalism is a relatively new idea in human existence, but the foundations of it are wired into our biology. In-group/loyalty memes are easily adopted because we naturally have kin bounds. Your tribe, state, religion, nation, etc are group identifications that expand the kin bond to either like minded people (religion, politics, etc.) or like fated (tribe, city, state, nation). Post Nationalism could occur by two means. A cultural pruning of the memes around in-group/loyalty (which is feasible yet unlikely) or an expansion of in-group/loyalty to include all people and potentially Earth bound life. We are all floating around on the same rock in space, and our existential problems affect every living thing on Earth, so the sooner we get to Post Nationalism, the better.