* Well, native speakers of the language. Not just anyone.
Speaking as a non-native speaker, I take issue with this. The price native English-speakers pay for the convenience of the widespread use of their language all over the world, is that they don't own the language any more.
I may not have been 100% clear, so let me explain my point better.
I don't mean to say non-native speakers do not influence a language at all. English has had huge influence on it throughout its history. In fact, every language has. And I very much subscribe to the idea (as I mentioned) that there is no "right" or "correct" language. Language is correct by virtue of being used a certain way. There is always a "standard" version of a language (or several standard versions, as the case may be), but this is strongly connected to social issues, to literary language, to perceived "appropriateness" of a certain form of language. This has nothing to do with one form of language being objectively correct. As I said, it's a tool of cultural imperialism more than anything else.
As a result, if a variant of a language (not just English) is used by a certain group or community, and it is "incorrect" in the prescriptivist sense, I think that notion is useless from any reasonable point of view. It's a way to paint a group of people as inferior to another group, there is no objective reason one, enshrined version of a language is "better". It's all about status. There are ways to use language appropriate for certain situations or social circles, but if you speak a language a certain way in a consistent way, it works.
But. Non-native speakers are a more complicated case, and there's a reason I made an exception regarding that. A person speaking a foreign language uses it differently, from my understanding of linguistics, than a person using it natively. It's more about learnt rules of using a language. This leaves room for errors, due to imperfect understanding of grammar or idioms or even vocabulary. It can be less about "that particular person speaks a certain way" and more of a "this person has problems following the patterns of the language".
This is not clear-cut, because a person well-acquainted with a language is, to my mind, much closer to a native speaker than to a language learner after passing a certain threshold. English is not native for me, but it's natural enough for me to think in English. I don't mimic the way English works, I use it naturally and with ease. I imagine this is the same way for you. And most if not all non-native users of English in this forum, for that matter.
But, lines are much more blurred with non-native speakers. I did not mean to imply the development of the English language is solely on 100% purely native speakers. But I am more wary of putting learned English on the same level as naturally acquired English, because even if there are gray areas, some less experienced speakers of English clearly speak it in a way that is not "correct" in the sense of following the way the language works. Using a grammar form incorrectly or misunderstanding an idiom by a language learner is not necessarily just a pecularity of their speech, it *can* be regarded as an error. For a native speaker using a form consistently, I reject the notion that they are *capable* of making an error in this manner. If a native speaker speaks in a certain way, this way is, in my view, correct pretty much by definition. For a non-native, it... weeeeeell, it might be or might not be. It's complicated. That's why I avoid taking this "anything goes" approach when talking about a non-native speaker. I hope that makes any sense.
The same ambiguity goes for groups of people. Indian English is not *technically* native for most of its users, but it's distinct enough and has repeatable patterns to a large enough extent that it's a borderline case of an actual English dialect. Same goes for the way English is used in much of Asia. I hesitate to say that's the case for Europe, yet.
But there's no clear divide here, so I am cautious about thinking of English being used non-natively by a group (based on their ethnicity, nationality, geographic area) as a kind of fully, for the lack of a better word, legitimate English. After a long enough time, it is. But the time where the transition of "a group of people use a foreign language and they misuse it in a certain way" to "a group of people use a language and they make it their own" happens is difficult to pinpoint. It's not clear when a dialect is already there as its own thing. In fact, it's impossible, like many things in linguistics, to define that in a sharp way.
And, I should point out that this is a point of contention, especially among linguists.
The existence of long-established variant forms of English (American, Australian, British, Canadian, Jamaican etc.) already disproves the notion that all native-speakers agree on what is correct. (...)
That's just the thing. ALL the forms of English that you mention are, in my view, 100% correct, by virtue of being forms of English used natively. My "if a native speaker says it, it's automatically correct" belief is very common-sense to me, because I see it as logical and at least approaching objectivity in the sense that I get rid of elitist, cultural interference of an arbitrary "you can't speak like THAT, it's not proper" that English has struggled with much more than some other major languages. But this "anything goes", as much sense as it makes for me, in itself is controversial, and I know many would think the notion ridiculous. The level to which non-native use of a language is legitimate and "correct" is bound to be even more controversial and complicated. Again, that's why I avoid making a categorical statement of "everything is correct", the way I do with native speakers of any language.