> If the grammar rules you learned in school disagree with (any!) native speaker, the rules are wrong.
I understand the sentiment of "the language is defined by its speakers", but this statement seems a bit overblown. According to that logic, it is literally impossible for someone to be incorrect about the meaning of a word.
> it is literally impossible for someone to be incorrect about the meaning of a word.
Yeah, it's important to frame it in terms of idiolects and dialects—any given speaker has an idiolect, and that idiolect is worth describing and documenting uncritically. But that speaker also benefits from speaking a shared dialect with other speakers, and it's valuable for that speaker to be on the same page with other speakers of their dialect about definitions.
I think what OP is getting at is that it's not the role of linguistics to assign a value judgement to a given usage—there are merely benefits that speakers can derive from better understanding the dialects that they use in daily life.
I understand the sentiment of "the language is defined by its speakers", but this statement seems a bit overblown. According to that logic, it is literally impossible for someone to be incorrect about the meaning of a word.