But on the whole, there do have to be rules for grammar, just as there have to be ones for spelling and meaning. otherwise we are all just a bunch of Humpty Dumpties who cannot effectively communicate with one another.
I imagine grammarians face the same dilemmas as lexicographers - once upon a time they could argue that their role was to describe not prescribe or proscribe, but starting some time ago (long before any of us here were born) there have been a couple of very important innovations which affect the validity of the theory that language just changes and nobody should ever try to resist that.
Education and dictionaries.
Once everybody is taught what existing words mean, and how they are spelled and used, and their meaning etc is documented, it becomes a lot harder to say that "wrong" is just "evolution", and much easier and valid to argue that if, for example, 'electrocute' is defined to mean a fatal electric shock then to start using it to mean a non-fatal one is simply wrong, and remains wrong no matter how many people are wrong.