I think that dialect is more probably left over from previous versions of the language or other languages (and not achieved nationwide usage).
I presume that you do not think that all the (essentially very similar) variants/dialects of English arose spontaneously and independently 'from scratch' - since that would be a ridiculous suggestion. They have clearly all 'evolved' from some ancient 'core language'. That being the case, the development of all variants/dialects (well, at least all but one of them) must have been initially the result of what you would call 'incorrect' use of the current language.
Rubbish. How can using words which actually mean the opposite be a result of that.
I'm not suggesting that it is. In what you wrote originally, you seemed to be implying that "terrible" was an acceptable word for "bad" or "very/extremely bad". However, although dictionaries have now succumbed to that new meaning (with, I imagine, your disapproval) what aspect of the telephone line to which you referred was in any sense related to terror?
Did someone one day get fed up using "very" and decide "terribly" would be a good alternative?
As I said, I don't think that, in most cases, it was anything to do with people having got fed up with "very" - rather, they perceived the need for a wider range of degrees of "very" than was available in existing words. One problem is that different people have different perceptions of the 'ordering' of the list of words they now use - but I think most would say that words like "terribly", "awfully", "incredibly", "extremely" etc. are probably 'stronger' than just "very".
Maybe you're exceptional, but I would be very surprised if you do not, at least sometimes, use words like "terribly", "awfully", "incredibly", "extremely" etc. when you do not really intend to imply anything to do with terror, awe, non-credibility, true 'extremes' etc.
You will presumably embrace the use of "so" to start every sentence, "like he was..." to mean "he said" and other such unthinking, meaningless drivel.
Not at all. Not the least because, in general, I am very 'conservative' and resistant to any change, I hate all such things. However, I accept that in a few decades time (when few of us will be around!) some of these things may well have come to be regarded as the currently 'correct' language.
As I always say, if you turn the clocks back a hundred or five years, there were undoubtedly people like you who were passionately opposing some of the (then) 'incorrect' uses of language which, with the fullness of time, have come to be what you and I would now regard as the 'correct' use of the English language. Like it or not, a lot of what we now regard as 'correct' would have been regarded as incorrect (and maybe despicable!) by Chaucer, Shakespeare, Dickens or even our grandparents.
Kind Regards, John