*cracks his fingers. Pushes up his glasses.*
That's not my point at all. Yes 70 years have passed, but it's two different things you're talking about. Technology as we know, is easier to innovate and upgrade. Why? Because once it's out there, it just requires mass production and physical labor. However, in the world of bending, such things come at such a slow rate, that an explosion following it seems a bit...inconsistent. Bending is more spiritual, and it's hard to think an artform that was just discovered (metal), and lightning (super rare) is now just some mainstream do-hicky thing that can be done by snapping your fingers.
How is it inconsistent? Toph founded a
public school specifically for Earthbenders to help perfect and improve upon their Bending, to achieve a state where they could bend Metal as well as Earth. In a world based around these supernatural martial arts, how is it unrealistic that this kinda knowledge could be disseminated to the Earthbenders at large? Regardless of the sorta characters you stumble across in this series (even the mobster-style Firebenders popping up in Korra), Benders of all stripes have this sort of initiation into this "martial arts subculture." A lot of stuff stems from the whole teacher-student relationship. There's a baseline level of spiritual awareness inherent as well; if they didn't have it, well, they wouldn't be Benders at all. The Earthbenders have always had the potential to bend Metal thanks to the impurities of Earth extant in it; Toph just gave Earthbenders the means to do so by imparting her wisdom and her knowledge of it. Also, 70 years is a long *** time to train quite a few Metalbenders, who would in turn train more Metalbenders,
ad nauseam.
As for "everybody being able to channel lightning...?" Consider this: What was the Fire Nation like around 100 something-odd years ago? Positively ****ing warlike. Aggressive. Most of your Firebenders then had little patience to meditate on their art, and they almost always took the most immediate path to power (which was just flinging fire everywhere). They were characterized by being rash, impulsive, and prone to fighting at the drop of a hat, all things that assisted in Firebending but very limiting at the same time. It clouded their spiritual enlightenment; Iroh himself said that manipulating lightning was rare for that reason, because not a lot of Firebenders can achieve that state of Zen-like calm to muster their energies.
Fast forward to the present: The world's a (more) peaceful place, and the Fire Nation is no more. There's a lot less aggression and a lot more time to focus on the introspective parts of the art. Plus, I'm sure it didn't hurt that Zuko's resolve to spreading peace and co-existence between the denizens of the world and the Fire Nation also clued Firebenders in to his revelation at the hands of the last two Dragons, that Fire is a force of creation and life, not just destruction.
Just think how hard it was for Toph to achieve metal bending. She was in a desperate situation, and not only because of that desperation, her blindness aided in her being able to bend metal. Given there's only been one generation that has passed (Toph's daughter), I just have a hard time imagining how metal bending became so easy. Even if she did open up a school, just the dedication and practice...and then you consider how anyone in the task force can do it just makes it seem like it's something that simple and easy to perform.
Simple phrase: World of supernatural martial arts. Read my first paragraph.
Smooth Criminal