Borderline of Correct and Incorrect
There is nothing which is correct or incorrect in language. Terms like correct and incorrect are relative and only make sense in relation to specific contexts. Something may be correct in one time or setting but be totally unacceptable or inappropriate in another. Often what is “correct” seems to be the opposite of “informal/conversationalist style”. Sometimes the degree of informality is so high that it constitutes a borderline to incorrectness. There are examples in many languages as for example the use of dative (which is still regarded as incorrect) instead of genitive in German. This informal/conversationalist style will stay peripheral for some time until it is able to make the first step to informality and then the next step to standard. This was the case with words like "ain't" in American English in 1966 when it was adopted by Webster. A lot of people complained and were afraid that this will degenerate American English and will support chaos. The contraction "ain't" has had an interesting evolution but is still restricted to very informal use. Consider "is not", which becomes "isn't", and "are not", which becomes "aren't". What the contraction is of "am I not"? We usually say, illogically, "aren't I?" and perhaps some Irish and Scottish speakers say "amn't I?"; in Victorian English it was "ain't I?". The trouble was that it was then extended to all persons of "to be" and "to have", was therefore considered sloppy and passed out of "correct" English usage. For instance a continuum or a scale is used when showing the degree of intensity, size (very large – large – small – very small), age and probability. A degree of formality on a scale or a continuum can show how words move both ways ie ascending or descending order from extreme formal to extreme informal and the other way round. Usually the following process is observed:
Incorrect - very informal - natural - standard - formal - very formal - old fashioned - outdated.
However, this process is not a flow chart of making a product and cannot be predicted so easily. It is as fickle as those human beings who speak it. The register of formality or regionalism (Ame vs. BE) cannot keep its position on the scale permanently. People often forget that language is organic and thus subject to change. This is language dynamism comparable with the process of human birth and death.