People seem to believe that just because this era is relatively peaceful, it means that no problems exist in the world and that there will never be any problems again.
1. No, all the nations are not united. The 5 great nations seem to be in agreement, but there are other smaller countries such as the secretive Amegakure, Kusagakure (the grass village), Otogakure (the sound village), Takigakure (the waterfall village), etc...
2. Humans will still fight over things like money, natural resources, etc...
3. Shinobi and ninjutsu still have other purposes such as putting out fires, finding lost pets, etc...
4. World peace wouldn't get rid of common criminals like normal bank robbers, purse snatchers, shop lifters, etc...