Sustenance is done by utility. The organic evolution of languages at the word level is determined by word choice, and every time we utter anything, by exercisi

Sustenance is done by utility. The organic evolution of languages at the word level is determined by word choice, and every time we utter anything, by exercising choice we're contributing to the sustenance of the expressions we just used. It's the same at the language level. I could choose to speak English or Japanese, but I can never speak both simultaneously at any moment. Choice is mandatory. People choosing to speak English is the force that corresponds to the greater sustenance of English, or any language X. From experience, I know that I do not prefer English at home or with family. So I will speak Japanese until I die. And I see this often. Most bilingual families speak non-English at home (privately), but speak English outside of home (publicly). This works extremely well, and we have no problem speaking multiple languages as long as there are enough natural and practical use cases throughout our lives. But had I been born speaking only one language, then this choice does not exist. And since languages are really hard, unless you're immersed in language usage, transferring a language in its entirety through plain education is impossible. That's why languages die off with their communities and their cultures, and last just a while longer academically until the last speaking person dies. Or, in the case of second and third generation immigrants, they don't bring the culture with them, so family usage doesn't entail everything that language entails. You can't suddenly speak Japanese in a professional setting if all you've done is speak it with mom. So this limits usage, and is the beginning of the end. With less usage, there is less sustenance.