@AngelofOctober Calm down. I'm certainly not going to throw you out.
Think of it like Japan or China. The cities have modernised, but the culture still runs strongly in the people and the rural areas are almost unchanged. Rosai's people are practically the same as they were in Aang's time, let alone Korra's. You have people, youth, who are embracing new styles and technology, but you also have the old or more spiritual people, who maintain the traditions.
Air nomads still fly on bison and glide in wing suits/traditional glider staffs. The Earth Federation, despite their political changes, are still varied between farmers, cityfolk and others. The Fire Nation would have changed the least, and the Water Tribes have probably established a trading route or tourism industry through the spirit portals. Only the United Republic would likely have enough infrastructure to support modern tech and vehicles as we know it.
And don't forget the influence of the spirits on technology. Dark Spirits in LoK destroyed mecha and snowmobiles, so a spirit portal and wilds in the middle of the biggest city in the world probably slowed down technology to the point where a computer might be more of a risk than it's worth developing. There's no space programme, so no GPS, no internet, no satellite phones, and no intercontinental warfare. You want to beat someone's face in, you still need to be able to see the bloke. Hence, bending.
In our case, bending is not obsolete, but nonbenders can match its power easily with chi-blocking lessons or tasers. That stops or restricts bending triads from exploiting nonbenders. An Avatar was never a military or temporal authority, but a balancing factor. If there's one person capable of stomping battleships into pulp or worse, war suddenly becomes a lot less attractive even if you're overwhelmingly stronger. Rosai may even be expected to learn the "fifth" element of chi-blocking or "nonbending".
So you're worried that the Avatar world is too similar to the modern world to be interesting? I assure you, it's close enough that we don't need lessons on imperial China to roleplay properly, but it's different enough that we aren't going to worry about Google Earth giving away the Avatar's position to hackers. And anyway, the Avatar is the target, not the solution. That's why there's a Team Avatar.