English isn't a formal language so it doesn't matter if there's no formal way of deciding grammaticality.
Usage is mostly about imprecise, changing social conventions.
That doesn't imply that it's useless to talk about it. We can talk about language usage more or less in the same way we talk about what to wear on such and such occasion.
That's not how I think about things. If you claim that a certain statement is "incorrect" English, to me that means you must have a decision procedure in mind. Sure, most of the time it's "common sense" whether something is grammatical: "The sky is blue" is fine and "They sky are blue" is wrong. However, the same applies to deciding whether programs halt or not. Most programs either halt quickly or are obviously infinite loops, yet we know a decision procedure that works on arbitrary programs is impossible.
I'm not suggesting that talking about grammar is impossible or pointless. I'm saying that for the discussion to make sense, there needs to be some formal way to settle whether something is grammatical or not. You could try to make a formal grammar for English, but that's probably futile. A better approach would be to choose a list of written works as the English "canon" and judge usage (hopefully as objectively as you can) against that body of work.
Usage is mostly about imprecise, changing social conventions. That doesn't imply that it's useless to talk about it. We can talk about language usage more or less in the same way we talk about what to wear on such and such occasion.