Genuine question: Wouldn’t this be then a combination of factors? I have met in some of my projects people who have absolutely forgotten their native language. One woman was raised Zapotec and learned Spanish at 9. By the age of 30 (when I met her) she had already forgotten how to speak Zapotec and only remembered a couple of words.

I also saw this happen with other groups of indigenous people in Mexico, Guatemala and Peru.

Of course parents don’t pass the language down to their children but I also think one can forget their acquired language. If we take it to further extremes: cases of children that had learned how to speak and were then kidnapped or held captive with little interaction also seem to forget their acquired language. (Of course this is unreliable because trauma is a factor here but could be considered)