For the record, here are the chief exploits of the past avatars according to the show:
Avatar Wan: Once Vaatu was vanquished, it's unclear how many wars he fought in, but I remember the dialogue saying it was several. If I'm wrong, someone feel free to correct me.
Known Military exploits: "several"
Avatar Yangchen: We only saw her advise Aang. According to other material, she fought a dark spirit, which I don't count because we don't know what that entails. The airbenders apparently made a festival out of it, but since when do pacifistic monks make national holidays over military conquests? Whatever, I'm counting the show only anyway.
Known military exploits: 0
Avatar Kuruk: Known for his lack of responsibility. He went around and fought a lot of agni kai, neglecting his duties. He lost his lovers face back to Koh the face stealer.
Known military exploits: 0
Avatar Kyoshi: Stopped Chin the Conqueror.
Known Military exploits: 1.
Avatar Roku: His life is defined by a lack of military exploits. He stopped the Sozin before a real invasion became necessary, then just lived peacefully since then as far as we can see.
Known Military Exploits: 0
Avatar Aang: It's hard to quantify this one. On one hand, he was part of several operations against the fire nation. On the other hand, his only main goal was just stopping the fire nation, so you could say those were just several skirmishes in one ongoing conflict. I'll count it as 3, if only because there are only 3 instances where the a significant portion of the population was in danger: The Northern Water Tribe invasion, the attack on Ba Sing Se lead by Azula, and attack on Ba Sing Se lead by Ozai. And it should be noted that Aang, of all avatars, considered himself a mediator first and a warrior second.
Known military exploits: 3
Avatar Korra: There's amon, UnaVaatu, and now presumably this Anti-Krew.
Known military exploits: 2, pending on 3.
Total military exploits in the last 6 avatars: 7, with half of them not experiencing a single need for military exploitation in their lives. Aang was unique in that he became active in the middle of the worst war the world has ever seen. Korra and Wan are the only ones who have attracted multiple different national level threats in their lifetimes.
So I hope that's proof that while the avatar is indeed a warrior, that's not what they've been known for being. If anything, it's a notion invented by Legend of Korra.
Again and do what? Commit genocide or cripple a nation to a point that allows another nation to swoop in for the kill once the Avatar is down?
The Avatar has been a mediator that was able to perform it's duties mostly because there was no opposition in the world that was able to rival them. Plenty of times armies, nations and individuals tried to contest that power, and every time the respective Avatar put them in their place... but technology has thrown off that power balance to a point where it's simply ridiculous to expect the Avatar to be a mediator simply by relying on their traditional role as an impartial ass kicker.
I'm not saying the Avatar doesn't still have the power to end every conflict, but the violence and brutality needed to do so in Korras time is on such a scale that it would make Zozins genocide look like a luke warm fart.
What? Why are you skipping to the avatar doing national level damage? Just replace the guy in charge if he is dangerous with someone who is not. Like they've always done when someone is too out of line. And again, the avatar is powerful enough that there is still nothing the modern world can compete with. What weapons are you referring to that can supposedly make the avatar obsolete? The only thing we've seen is mechs, which normal benders can take care of, electric gloves, which dont' give much more of an advantage, and cannons that can fire stuff slightly harder than people can from their hands. Which of any of these is going to make someone who can chunk a continent somewhere even blink?
And for the record, the avatar did not immediately use fear to make mediation work. Look at how people treat the avatar throughout TLA. Instant respect, whether they are common born peasants or nobles. The only ones who don't are people who are against him or that he later offends in some way. Respect for the avatar went a long way in politics back then. It's only in Korra that rulers (The council of book 1, the president, the queen) don't really respect nowadays.
