It might not fly because Grandma is a stubborn old goat, not because his argument is wrong.
That's a mighty powerful counterargument to any points she could make, hahaha.
Commonality is not the same as a "strict rule."
And no one has claimed that it is. Still, a strategy being frequently employed attests to it being capable of producing results desirable to its users, or at least to it being the optimal one out of the options on the table for them.
Japan had a thriving economy until the 90s, then the bubble burst. They went from a time when mimicking Western chauvinism and supporting a family with one paycheck worked, to a time when many workers could barely support themselves on their paycheck.
Japan is still a high-income industrially developed society with a high HDI, and thus a target for as much labour migration as they allow (and then some). Birth rates are still high within marriage, which means married households typically make enough to support not only the spouses, but their children as well.
moral norms only exist within extant power structures, and people often violate them when the context allows or encourages it. Unlike like physical rules, moral rules do not exist without a hierarchy to enforce them.
Peer pressure needs no hierarchy or power structures to enforce moral norms, and there are plenty examples in Japanese storytelling of the very extreme forms such pressure can take.
But there are differences when you change the scope of "public." World consensus? Country consensus? Household consensus? You get different answers to what is normal if you don't force the generalized metanarrative.
Moral norms are culture-specific, oy-vey. Grandma's also not arguing norms as narrowly applied within their household, for some reason or other, as if she wants Tsubasa to successfully operate outside it.
She has already lived for over fifty years, and her sense of norms has not kept up with the times. She is arguing for her specific worldview to be regarded as the norm, even though for many people it is not the norm. Her justification is her position of authority, not a meaningful description of society. [...]
She has spent decades being extremely successful within a particular social strata, and it only makes sense for her to know the norms there well. Unlike, needless to say, either Sonoda or, to a lesser degree, Tsubasa.
If you presume she is arguing in good faith - and there is no reason to think she doesn't, given that it's all for her granddaughter's benefit, in the end - what she is doing is sharing her experience of these norms with these less acquainted with them.
Every culture is one out of many cultures. And within every culture are many subcultures.
No, every culture is a thing in itself, so it's absolutely irrelevant for these within it whether its moral norms coincide with those of 90% others, 10% or 0.01%.
As for subcultures - sure, yet see above - if anything, Grandma's much more knowledgeable on how things are done in the circles she wants to bring Tsubasa into.
Complexity is not irrelevant. Even your "sky is blue" example has real-world implications, because sometimes the sky IS dark or cloudy and it changes our behavior. When the sky is blue and it is sunny you might wear sunglasses, when it's cloudy and raining or nighttime you probably don't wear sunglasses. You would be silly if you told someone to wear sunglasses at night "because the sky is blue."
Well, as promised: "Get off the stage, boo".
Not only is this "akshuly"-argument nitpicking (and thus serves as a demonstration of the faults of the original argumentation, trying to disprove a large-scale generalization with singular exceptions), it is also based on a faulty assumption ("sunglasses are worn or not worn because the sky is blue or not" - were the sky coloured green, purple or plaid, you'd just as well wear sunglasses wherever the sun was shining brightly on it).
It's
Derrida's concept of linguistic heirarchal pairs; "Life" is understood in opposition to "death," and any mention of one contains the other.
Typical humanities pseudointellectualism, made up of unverifiable and unfalsifiable fantasies. No further comment on this.