Well, I'm not entirely sure I would agree about the decrease in family sizes. Because Korra's environment is based on 1920s US / Shanghai, we take into account a lot of historical milestones in terms of technology from not just that era, but from those locations. Obviously, there will be a few adjustments based on S1 canon (since we're disregarding anything beyond), but for the most part your rule of thumb should be the 1920s. As far as bending progress specifically with regards to "pure" metals being able to stop earthbending — I've always, to be honest, found that a rather hamfisted deus ex machina way to limit the power of seismic sense. I know it was introduced in S1 but I'd have to check with Gia if she wants to keep that.

Family sizes at this time were still probably fairly large for the average family, especially for the poor. In the URN's socioeconomic brackets, in particular for Republic City, poor nonbenders would likely have a lot of kids for two reasons: 1) birth control / family planning in the 1920s wasn't accessible too readily for those without financial means (not to mention there might be stigma against it, depending on someone's beliefs), and 2) while more children = more mouths to feed, that also means more income since the work can be split amongst the parents as well as any children old enough (and in the 1920s, that's a pretty young age depending on the type of work). Nobility would likely think more along terms of a nuclear family, but very 'heir and a spare' and any children after that are by choice or accident.

I do wholeheartedly agree that the blue bloods would absolutely have prejudices against self-made wealth. I don't think all would think they were dishonest, but the aristocracy by and large would likely think that self-made people aren't as refined, and are still... how to say... the same commoner underneath all the glitz and glam of money.

I would agree, somewhat, that the advances in bending (and the more wide-spread teachings of each unique "special" ability) would take some of the mystique away, but I also feel there would be drawbacks to that. Mass produced doesn't necessarily ensure quality. While a bender may have the ability to generate electricity for their job at the powerplant, or bend metal for their job at the RCPD, doesn't necessarily mean that they would be masters of that technique on a daily basis never mind in a fight. I find it hard to imagine that canon exceptions aside (*stares at Mako*), the average 16 year old would have such a command of lightning so as to give master benders a run for their money.