I'm only on page 3 so apologies if this has been said.
fourfried the reason few posters are accepting your argument is you're mostly not talking to other language teachers. Whilst I completely agree that you're right, (having also studied language teaching and linguistics at a fairly high academic level and taught for 10+ years) with every respect you're not explaining it well!
Here's my attempt. Grammar can be prescriptive (tells you how to speak) descriptive (describes how people speak) or pedagogic (simplified for teaching purposes). Four fried is talking about descriptive grammar, which in academic / professional circles is the only kind people are interested in. Most of the rest of you are talking about prescriptive grammar which is beloved by pedants and derided by most people who've really thought about it.
If you are a prescriptive grammarian that means you think there are some things that are correct and others that are not. Or in other words there are rules of a language. It follows that we can ask who makes these rules? Unlike (e.g.) Italy, we have no academy of language whose job it is to decide what is English and what is not. So it must be the speakers of English then. So who are they? Well, they are many people. And they all speak pretty differently. And in this day and age none of us like to think ourselves arrogant enough to believe that our way is THE way. There are many ways. If a community of people speak a certain way, then that phrasing is correct in that community. No one community is regarded in the linguistic world as taking precedence over the others. It just doesn't stack up.
As one pp said, languages are alive, they grow and change. You don't have to go back very far to see evidence of that. If you want to see how things have changed almost beyond recognition you only need to look to Shakespeare or the King James Bible, both of which were instrumental texts in the standardisation of written English. These two books were essentially the gold standard of the language in their day. Read them now. Borderline unrecognisable as modern day English. Well no one has approved those changes. They've just happened, and will continue to. This is why people who've studied language are not much interested in descriptive grammar. It tells us nothing really.
A few further points!
I mentioned also pedagogic grammar. How can you teach something that's never right or wrong? It's very difficult to do obviously. So much in the same way as other disciplines (e.g. Physics) are simplified for teaching purposes, the same thing happens with English. We (as language teachers for speakers of other languages) teach a version of the English we speak, simplified for the level of the learner.
Usually (not always) this is based on what we might call 'standard' English, or in my case 'standard British English'. The difference between standard and correct is important though. Standard describes the variety of the language I speak. Something can be correct or incorrect within that. Someone who speaks a different variety (e.g standard American English, Scottish English) is not incorrect for speaking that way.
Does that mean speakers of non-standard varieties, or low-status varieties don't get judged by speakers of prestige-varieties? Of course it does not, but that is because people in general have no idea about all this stuff.
One final thing then I promise I'm done. Someone asked if speaking and writing are the same. The short answer is no. Writing 'they're' when you mean 'their' is not correct in any variety. Or perhaps more accurately, the above applied for the most part to spoken language. The written language is for obvious reasons much more standardised. People can and do choose to write in a variety closer to their own speech for expressive / literary / political reasons but for the most part in writing we use and are expected to use something approximating the standard variety of the country were in.
Op don't ever feel judged for your English. It's an amazing thing to have two functional languages in your brain and sadly most British people will never understand that.