The nations/states we currently have really are the best all things considered, not in their current state but the concept is right. If we don't preserve them our future won't look bright. Leftists don't seem to understand that what follows the "classical nations" are superstates and nothing else. If it comes to that you can wave goodbye to individualism, privacy and freedom. Just read 1984.
I don't see how 1984 cannot happen in a nationstate today. Neither can I find reasons why it would be more likely with a world government.
It won't bring the utopia you may suppose. There will still be groups of haves and have nots. There will still be government. And most irreducibly, there will still be human nature.
Yes, but eliminating one problem does make it easier to solve the others (those that are possible to solve anyway). Your argument would be appliable to feodalism, or anything really. Why are we not serfs under lords under lords under lords ? Because people obviously didn't like it (of course it's more complicated than just that but you get the picture).
Nationstates can by definition not deal with problems that are bigger than nations, and we do have that kind of problems today. Right now nobody has the power to effectively do damage control on climate change. Rather, we are hundreds of nations each with their own goals that are all interested in letting others solve that problem instead because it is from their point of view more rational. It's natural to be selfish, and we need political structures to avoid selfishness when it leads to ruin for everybody. Similarly, no nationstate can deal with a financial crisis or tax evasion or trafficking of drugs, weapons or humans even though it hurts all nations. People that cling on the idea that independent nations are an effective power structure today are simply being delusional. The only reason to maintain that mascarade is selfishness or a romantic attachment to your own nation.
In order to make efficient decisions, we need organizations that are not defined by their enemies.