It's the absurd use of the words 'correct' and 'wrong' in discussing language situations such as this that grates on me!
If an Eastender (I wasn't sure if I would be allowed, because of the opening four letters, to write the word I obviously mean!) says to his girl-friend, "Gerrup 'em apples!" is he 'wrong'? Of course not; he has made a perfect communication in a colloquial situation. That, surely, is the purpose of language.
Obviously, there are varieties of language that one would normally avoid in formal situations. At a marriage ceremony, one says, "I do" or "I will" or whatever and not, "Yeah." On the other hand, if a mate asks, "Are you going to the match on Saturday?" you are perfectly free to say "Yeah." In these circumstances, "Yeah" simply isn't wrong!
The British viper was originally called a nadder, but gradually the 'n' was peeled away from the name and tacked onto the 'a', giving 'an adder'. We've been living perfectly happily with that 'mistake' for 700 years...nobody thinks it's 'wrong'. As already pointed out, we've been doing exactly the same for ages when using pound as a plural, so it's not 'wrong' either!
I'll leave it at that.