"Surely one of the main features of English grammar and usage is that it is descriptive rather than prescriptive, and if sufficient people accept a certain usage, eventually it can become the norm?"

It "can," but if it doesn't, watch out.

Some forms have been in the language for a long time, and they are still considered ignorant. It's all very well to talk liberal, but if a person from a poor family wants to get a job that involves answering the phone or talking to customers, she better lose such things as 'ain't' 'We was' and double negatives.

Her employer may know she's smart and dedicated, but the customers will assume otherwise.

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Whenever I go to the library, I look for books in 420's, the books about English. I have to smile when I read books by professors which say that people are stuffy when they object to a split infinitive.

Yet the professor's book itself will have no split infinitives in it. Why? Because he needs to sound educated.