By this standard, absolutely nothing we say is grammatically correct, because our language is so modified from what the Post-Roman Germanic tribes who invaded the British Isles spoke that it’s no longer the same language, except maybe in a political sense.
That’s what gets me about peevers like you: You make an arbitrary mark in the imagined past and claim “this is correct language”, and nothing which happened before or since counts. The fact you lot get it wrong, and speak such transparent falsehoods when you attempt to discuss the history of any language living or dead, is secondary to the utter arbitrariness of the underlying concept.
Let’s imagine, for a moment, that you’re learning a new language. You hear native speakers of this language using a certain word over and over. Judging from the contexts, you suspect these people are using the word in the same sense, but you aren’t completely sure about the exact meaning. So naturally you consult a dictionary. Now, wouldn’t you prefer that the dictionary provide a definition for that sense of the word, so you can finally understand how people typically use it? Would it not frustrate you to find that the dictionary didn’t list any meanings for the word that seemed appropriate for those countless contexts in which you heard it? Or perhaps even to find that the dictionary didn’t list the word at all?