You are right that closest friends die, but let me ask you something. In the American Revolution, do you think that the British specifically targeted George Washington's friends? No! They targeted the whole army. As a result, George Washington did have living friends after the war (unlike the vision that you have for Naruto). You would be right in saying that the war has had only one death if the war was Akautski vs. Konoha 11. However it is not. It is Akautski vs. the 5 nations and the samurai. Therefore, Naruto's friends are not going to get special attention from the villains. This is why their survival is quite plausible even for a war.