Well lucky for Sasuke, Naruto wasn't in Sage Mode at the very end there or he'd be stoned right now.
Not sure that would have been a problem. Remember, Sasuke knows how to employ natural energy too, via the Cursed Seal and his recent power-up. In addition, he has Hashirama's cells.
Having constant conflict is not the only way the ninja system can live; its just what sasuke sees as the best way to fix the issues. A somewhat fair view point, but in all honesty, it stagnates the development of a nation and uses way to much fear on its citizens. I guess if you're that cynical you could argue its the only way, but this war has pretty much turned the ninja world on its head zzz. I'm sure that with the recent happenings (aka the war that almost wiped them all out), the gokages will be willing to implement policies for peace that will stick over time, even after they're dead.
It is the only way the current system can live, the series makes this clear. The villages are their nation's
military force, paid specifically to be their standing army. In addition to that, the villages get paid for taking missions, the most expensive of those being assassinations and espionage, things that can't/shouldn't exist in a peaceful world. If the villages don't take these missions, they will begin to dwindle: this is exactly what was happening with Suna in Part One. They were losing missions to Konoha and even their own Daimyo was beginning to depend on Konoha for work. They were going bankrupt (only being saved by the Kazekage's ability to make gold) and didn't have the resources to train their own forces, resulting in them having to make the most of the few they had. It was that reason they turnt Gaara into a Jinchuuriki, and it was that reason that they eventually decided to team up with Orochimaru and go to war with Konoha.
The point is, that if there is no longer any war, there is no longer any reason for their to be ninjas or ninja villages. There is no other point to the ninja villages except this - being their nation's military. That is literally what they train their children for from the start, to fight and kill. They don't have any other purpose. The Kages aren't anything more then generals, under the rule of the Daimyos. Even when the villages were at "peace", they were still plotting and working against each other, still building up arms in the case of Kumo and Iwa.
And Naruto will bring peace, not that it'll outright loose it once he's gone. I dont know how you got the implication it isn't possible after he dies, he just has to do his job.
Once Naruto has brought peace I'm sure other people can continue it.
For Naruto to bring peace, that would mean the elimination of the ninja system, and that doesn't appear to be what he's working towards. Jiraiya, Minato, and Nagato all pointed out that there can't be peace with the current system, and the Rikudou Sennin explained that ninjutsu was what was causing the problem. You can't have the ninja villages still going around fighting and killing each other and believe that there will be peace.
The thing is, maintaining peace is far harder then ending it. To maintain it, everyone has to work at it, but to end it only requires a single person. It's the whole idea behind the cycle of hatred.