Congratulations. "Science" has "proved" that there are no standards for language and all those teachers that marked up your papers with red pens were just being mean.
Your sentiment matches mine more or less, but I also think there's something that needs to be thrown into the debate: whether we should try to use grammatical/spelling/usage rules prescriptively is not really a question of science or history. You can say that attempts to prevent the evolution of language often fails, at least to some degree. You can point out that our current rules of "proper English" are not an absolute and unchanging set of rules that have "always been that way." However, none of that addresses the question of whether we should try to stick to a particular set of rules.
That's one of the things that people who are really into "descriptivism" as a theory often miss. Languages are dynamic, yes. There are various dialects with different rules, of course. Nobody is arguing about that. The question of whether we want to teach people to use the Oxford comma... well, that's something we get to decide, and not exactly a question for scientific study. To clarify: In trying to decide, we could devise a scientific study to attempt to find out which kind of comma usage is more confusing to more people, but we could still debate many other related issues.
Ultimately, if you write something like, "I should of gone to the store yesterday," it makes you look, at best, uneducated. Of course, we all have typos and little brain farts. And yes, if people keep saying "should of" and "could of", it may eventually be considered valid within some dialect. In fact, it's perfectly reasonable for an author to use that choice in dialog to convey the casualness with which someone is speaking, or the educational level of the speaker. However, until we have a whole separate dialect in which that's considered proper, it's supposed to be "should have" or "should've".
And that's another key issue here: these rules are contextual, but they are rules. You can have a dialect where the grammar is different, but then there are still rules, just a different set of rules. Slang usage can be different from "proper" usage, but even slang has a meaning and a set of rules that are understood and accepted.
I don't know that anyone is arguing that there should be a "one, true, proper English". However, in whichever context you're speaking English, there are rules for how you speak, and you should probably follow them. Knowing those rules might include knowing when to use slang, even when to break the "rules" in order to create an effect and clarify your meaning. But there are still rules.