When I started teaching in 2000 I was tasked with writing a grammar course, but in turning over the lit I found almost no grammar and a great deal of linguistics. This is not an accident: the two are exclusive or at least at loggerheads. Linguistics is descriptive while grammar is prescriptive and normative. John McWhorter puts it as bluntly as one can put it by saying that linguists look forward with eager anticipation to the day when we no longer care about or bother to enforce the prohibition against saying “Johnny and me went to the store” or “ten items or less” or “If a student has a problem, they need to see the director.”
All this iterates the is-ought distinction and will probably share in both its forcefulness and incoherence. The avoiding of oughts seems to be made even more reasonable by language being so much more conventional and fundamentally arbitrary than, say, morals. Even if we tried to set up the equivalent of a grammatical “natural law” that gave structure to a linguistic positive law the linguists would take it as a Chomskyan universal grammar, not an elevated language of discourse.
Linguistics has some room for elevated or solemn modes of discourse as opposed to vulgar ones, but this can’t be translated into grammar as even what counts as elevated will not be normative. Grammar ultimately has to be based on a canon and a historical norm, along with a sense that it specifies a sort of discourse that we reach for.