People would utter whatever they chose, and no one would understand them.

This exists. It's called glossolalia or speaking in tongues. For some it's a religious experience, and for others it's nonsense. I'm afraid that prescriptivists exaggerate in their own way similar to how descriptivists exaggerate, i.e., for rhetorical pruposes. Very few prescriptive rules have anything to do with ambiguity or clearness of meaning; many say they do, but they don't. The thing about correcting somebody's "grammar" is, that if you can correct it, you've already understood it, and what's the use. It simply annoys people. Somebody with aphasia or the desire to speaking in tongues doesn't need to be understood, nor can they.

There's something that's been studied by sociolinguists (mainly in Europe) called accommodation (theory). People speaking different dialects or languages who wish to communicate often accommodate towards one another. They end up not speaking their own dialect/language, but something in between. That is, if they wish to communicate. I've observed that many speaking the same language oftentimes don't.

keep that misusage out the authoritative representatives of the words of our language, i.e. dictionaries.

A dictionary is a book written by humans. Monolingual dictionaries are written for people who've already learned the language they're written in. In other words, nobody learns a language from a dictionary. Or if they do, it is not a language that anybody else shares with them. You need a grammar (in the technical sense) which is a set of rules for generating legal sentences / utterances in the language. (And you don't find grammars in books, you learn them from people speaking a language while growing up.) A dictionary (or lexicon) is not only a collection of vocabulary, but also the place where exceptions to rules are listed. Nobody would suggest that the plural of ox, i.e., oxen, is wrong, so why insist that ain't or irregardless is? In fact, ain't for the contraction of am not is OK.

This does not mean that I don't write in a standard register formally or that nobody should, but the web and forums thereon are not the place for formal writing. I don't for the most part dislike the the extra, tiny set of arbitrary rules for writing formal English so much as I dislike the prescriptivists who argue from authority (often anonymous) or logic, etc. (NB, the "arbitrary rules" above are different from the grammatical rules a speaker learns during language acquisition. Those rules are unconscious, and the arbitrary, prescriptivist rules are anything but.) There is absolutely nothing wrong with splitting an infinitive, ending a sentence with a preposition, or using which for a restrictive relative clause. Not a thing. Most of these rules were invented ex nihilo in the 18th or 19th century, and completely contradicted the grammar of English. (That's probably why they're so hard to follow; sort of like our spelling non-system.)