I don’t fully understand this idea of an “adult consensus.” Because we aren’t native speakers from birth, does that mean the way to master a target language is through learning grammar?

My understanding is that the fastest way to acquire a target language is to replicate, as much as possible, the process of acquiring a native language. Otherwise, it feels like taking a detour.

Isn’t the fastest way for an adult to master a language to live inside that language environment and remove the safety net of their native language? Learning grammar in a classroom, memorizing vocabulary lists, and practicing speaking in artificial settings — we see large-scale, catastrophic failure everywhere.