All true, but when you learn a language you have to learn it to a certain standard. There is still the language's "most correct form", which is, technically, the only "correct" way to make grammatical structures and to spell.
That said, languages do change. But not every new form of "speaking language" get "accepted in the correct form". No matter how you look at it, the sentence:
"Me and my man was going to the theater, then we run into dude i know"
is incorrect. The other person will understand what you said, the communication has worked, there is not a real problem. But it's still wrong in the end. When a language is updated, they don't add every new form or way people might use. That's first off impossible and second it would defeat the purpose of having a set of lingual standards in the first place!
Pronounciation is a different matter. It's more prone to be affected by the speaker's first tongue, dialect etc. Setting a standard for pronounciation will almost always effectively render a foreigners speech incorrect. Setting a standard for spelling and grammar does not.
My point is, languages evolve, i agree with you on that. And there's nothing wrong with that. As long as the point comes across the objective of the language is completed. But there is still the "ideal" of the language. The true, proper way to express yourself in English, Polish or Russian. And if you divert from that, it's still wrong at the end of the day