To be honest, them going from blimps to internal combustion engine I can understand when they recently developped a metal bending school, this would be a technological boom for them. Just like when internal combustion arrived for us but smaller than when computers became small enough we could go from dial ups to smartphones in less than two decades.
No, what I still cannot accept is the Korra canon that electricity for them comes mainy from a bunch of lghtning benders!
Since, you know, Azula was considered a genius for being able to do it and her older brother wasn't, even Iroh could not do it and just developed a counter. Could Ozai? (No seriously, I don't remember. But if he could I guess I would?)
Metal Bending was something Toph discovered due to her skills, but seems to be something anyone could do after she taught them. Just, you know, not as well.
Lightning Bending however was already known, just really really rare. And really really hard to learn. Yet they made a whole industry of those and no one imagined a better way?
People developing air bending out of nowhere is more believable than this.
Bending at the point of both series comes from genetics, after all (thus why the Avatar is so special), they just could not access theirs until something awakened it in them. I just finished Zero Time Dillema and it mentioned a few times something called epigenetics (a real study by the way, unlike some of the series' fields like the very mentioned metempsychosis, which exists but is not a studied scientiphic field, just a phylosophical one), that could explain this.
Basically, while our dna does not change through our life (save for damages like aging, radioactivity and terrible luck that can cause cancer) it has some triggers that say what will or not manifest, and those triggers can be turned on or off in situations of extreme danger or distress. In the games it is what can give the ability to get info from other routes, and in here would give the non-benders with air bending ancestors bending of their own.
But, you know, I kinda doubt they thought of it that much.