Ain't is a very commonly used word in the U.S. All parents and teachers have tried to teach their kids that it is improper English but it will not go away. As Sarab said, it is used more for effect than anything. Even well educated people use it informally. It has more effect to say "It just ain't gonna happen" than to say "It will not happen".
There are hundreds, maybe thousands, of songs that would simply not be the same without "ain't" in the lyrics. Bob Dylan's "It ain't me Babe" wouldn't sound right if it was sung with the words "It isn't me Babe". Fats Domino's "Ain't that a shame"; "Ain't no sunshine when she's gone", etc., etc. Phrases like "It ain't over 'til it's over" and others would justnot have the same impact.
It is understood that we all know that it is bad grammar but there are times when no other word will have the same effect. It is sort of like swearing. We could do without it but at times we just have to use it for emphasis.
As for "y'all", a contraction of "you all (you (plural), each of you, all of you)", it has made it into the dictionary as accepted English.