The Fire nation policy of genocide makes NO sense.
Bending is extremely valuable.
Air benders gives fast travel by air, ground and sea using sails. Imagine people riding across the roads in larger versions of these. https://en.wikipedia.org/wiki/Land_sailing
. Plus metal working needs air pushed around, and air can be used to clean easily.
Water benders can clean the pollution, push and pull water (like through a factory) and make transportation easier.
Earth benders can create infrastructure like roads, bridges and canals easily. They also have those railway systems.
They should have gone about a policy of collaborators. Offer citizenship and benefits to those that work for the fire nation. Kidnap younger ones and educate them as fire nation citizens. Offer POWs the choice between prison or work in labor battalions.
The Air nation didn't need to be genocided by pure killing. Instead the Fire nation, and collaborators captured the temples. A bunch of them flee into the countryside controlled by the earth states. Others accepted the rule of the Fire nation. 100 years later their culture has basically disappeared and they live like Fire, Earth or Water nation citizens. Cultural genocide.
It doesn't impact Aang's journey. He was already a master airbender.
It still gives him the lost people. But it also has him fight fellow Airbenders working for the fire nation or other factions.
Also it makes fights better as it has all elements against each other. So instead of Azula bumming around with a circus performer and a knife lady, she instead leads the best of the collaborators.