The Unfairness of Child Language Acquisition

We recently had a request for a community interpreter to help induct a child into a local preschool or kindergarten, and I had an opportunity to speak with some sojourners or immigrants who have placed children in educational institutions here and observed their linguistic development, as well as some locals who lived abroad for a few years and have just returned. The common refrain is that after a couple of years the little children speak Japanese far better than the parents (or, in the reverse pattern with locals who lived in English speaking countries, that the children speak English like natives and end up criticising their parents' Japanese accents).
Since it used to be a constant worry of mine that I would one day have children whose Japanese far surpassed my own, I made considerable effort to maintain my speaking aptitude at at least a high school level. (I'm not sure there is such a thing as college level. Most of my university students are usually hunched silently over their smartphones, evincing no oral communication ability in any language.) I considered cases in which mixed-marriage couples with varying degrees of ability in Japanese and some other language have tried, with varying degrees of success, to raise children bilingual or multilingual.
Now, as a pertinent aside, it was recently said to me that couples 'usually end up in the husband's language'. Being party to the contrary, my confirmation bias pushed me in the direction of outright disbelief, but just to be sure I put together little chart (which I will not reproduce here) with X and Y axes for the husband's and wife's native language, respectively, and plugged in all the mixed-marriage couples I have ever met in Japan. I found that, to my disappointment, for the couples composed of Japanese wife and English-native husband, the number of couples whose default language for communication was English is exactly double the number of those using Japanese. And where an English-native woman is married to a Japanese man, I find twice as many couples speaking Japanese as speaking English. So I'll be darned.
For couples composed of women from other Asian countries married to Japanese men, all those I've known seem to default to Japanese. I don't know any men from the Asian continent married to Japanese women. All the Chinese men I know, for example, are married to Chinese women. Exceptions doubtless exist somewhere in this country, just not in my personal acquaintance, limited as it is.
Interestingly, where one spouse speaks a Romance lanuage (Spanish, French, Italian, or Portuguese), the couple almost always speak Japanese at home. The only exception I've seen is where they speak English, which is not native to either spouse. I've found this to be true whether the Romance speaker is the husband or the wife. I assume this is because Romance languages are so much in the minority here.
Once the default language at home is established, one can observe the development of the hypothetical child's ability in Japanese--which is a given, considering their life outside the home is going to be entirely in Japanese--versus their ability in the language spoken at home if the language is something other than Japanese. I have yet to see a case in which children grow up able to speak a foreign language where at one of the parents is fluent in Japanese. Kids are smart enough to figure out when they don't actually need to speak more than one language, and they take the path of least resistance. It takes real dedication on the part of the parents to circumvent this, and I've known only a few families who succeeded.
Growing up in the US, I went to school with the children of immigrants, which children grew up with native-level fluency in two languages only because the parents spoke barely any English at all despite living there for decades, so that the children were forced to communicate solely in their parents' native languages at home and, obviously, solely in English outside the home. Where the parents spoke passable English, even if they refused to speak it at home in an attempt to force the children to preserve their linguistic roots, the children still answered in English even when spoken to in the parents' language.
Based on this limited observation, children seem to conform to the majority environment, and sometimes the only way to guarantee that children will grow up with native-level fluency in two different languages is for one of the parents to be unable to speak Japanese--and sometimes, even this isn't enough, and the child simply ignores the non-Japanese parent. Put another way, the only way to provide your own children with the advantage of bilingual fluency is--sometimes--to deny yourself that very advantage.
That is all. Anecdotes are not evidence.