In ideal situation I would have said that removing all borders will reduce major conflicts. But we don't live in an ideal world. The "ruling class" will not want to let go of the borders for political reasons. But removing social borders(i.e. allowing people to move across the world unhindered) will help disconnect the masses from the classes. Most of the major wars are started by the classes and millions from the masses die due to that. If the social borders are removed, it will be very difficult for the classes to drag the masses into conflicts due to political reasons/borders. In that case the wars are then relegated to a very small section of the society and hence will be limited and of smaller duration.
with political borders comes jurisdiction, so even if social borders are removed there won't be problem of criminals going unpunished. We do have present structures such as local justice system, international courts etc in place to tackle those issues.
Practically you run into so many issues, for instance in California the residents are opposed to building more housing. Young people in the US might not want to get a college education if they have to pay an order of magnitude more for college than people do in some other country. There is really a lot of fear that employers will take advantage of labor that works for less, etc.
On top of all that there is the fact that people are stressed by the fact that they”re going to die in a world very different from the one they are born in, often people find it hard to take that things have changed and certainly a changing population is part of that.
1. People should not be interchangeable. Polish hold different values than Cuban. They have their own languages, cultures, values, traits, physical appearances. Keeping borders is also making the world richer. I will be saddened for the rest of my life because I will never experience a massive culture shock like navigators experiences a few centuries ago. Now, I can pretty much have raclette anywhere, Starbucks, McDonalds, KFC, coke, Netflix, Hollywood is pretty much everywhere, and it's sad.
2. If we let go of the borders, we will give up on a vast amount of literature. Fiodor Dostoievski's masterpiece would have never existed if 40% of his population were French, Italians, Nigerian, Polish, Germans.
3. People would just abuse the system, using highly capitalist, low tax countries during their youth and move to socialist countries as they as getting older (of course that does not work, you need borders for this) and it's utterly unfair to let people from outside abuse a contributive system.