I think kartal was referring to "human-made and upper-class-imposed rules of so-called proper language" rather than the inherent connections between words that form a sentence.
But since we are dealing with such a universal and therefore complex being as the human being, "meaning" has an influence on grammar: as humans change, so does the meaning of words, hence grammar changes with it. However, while it is only natural for language to change it is also natural for human beings to attempt to create an understanding of what the "meaning" of words is, thereby creating those supposedly fixed rules.
I re-read
a book that also touches this topic, and it also has all those linguistic terms and references I cannot provide. It mostly deals with how the brain dealing with language is similar to it dealing with math. The author makes a point of differentiating between math and calculating numbers.
That being sad, I think those rules of grammar are not too bad to have. They might be an agreement of aesthetics, of what is proper, and certainly can be abused. However, I also think they are inevitable. One side, to take up the separatist argument, decides for one "meaning" to be proper, while the other side defines it differently. One person will quickly learn either, when exposed to those groups. You can't have "meaning" without "context", so if the context is "parents" your language will be different from when the context is your soccer team. For example, "to can" has different meanings, depending on context.
In short: when context changes, so does language.