After the Viking invasions and the settlement of the Danelaw, only the kingdom of Wessex in the south offered a center from which to publish things in Old English. Alfred the Great started the project of translating great works into the English of his day. The Anglo-Saxon Chronicle got started. By the time of Edgar the Peaceable, England was united. Short legal documents, called writs, were being published by the government in English. Most of the OE documents we have are from this period and they are in the heavily-diphthongized West Saxon dialect. When we think of OE today, we think of WS Old English. It was the "proper" English of its day.
The Danish settlement had already begun to change the Anglian dialects of Old English, however, adding much new vocabulary and simplifying inflections. After the Norman Conquest, when nobody who was anybody spoke or wrote English for a couple hundred years, the WS dialect lost all status. When English arose again and began to publish, the new sound of English was the West Midlands dialect of Sir Gawain and Pearl. The Scots borders also contributed to this rising status of English.
But the future of Middle English wasn't in the West or the North. It was in London, where commercial power lay. And it was in the now English-speaking court, where Geoffrey Chaucer was writing. Chaucer uses the London dialect, heavily laced with the Frenchified vocabulary of the aristocracy. This is the basis of today's English.
Still, it wasn't the accent of London that won out in the end. It was the language of the universities that became what we call today "Received Standard" or "the Queen's English." Birth and achievement made men rise to prominence in the England of the Middle Ages and Renaissance. The idea that a university education was essential as a mark of breeding for the aristocracy and doubly essential as a mark of fitness for responsibility for the lower gentry and middle classes took a long while to catch hold. But catch hold it finally did, so much that grocer's daughters like Margaret Thatcher and representatives of the working classes like Tony Blair were sent to school to learn the accent that makes them acceptable as leaders of government and society.
The most recent chapter in this story of low status dialects replacing high status dialects is the rise of American English. Despised in colonial and early federal times by visiting Brits, mocked by those aspiring to the upper crust within living memory, American English has replaced British English as the standard of English around the world. All over the world, even in former British colonies, people want to learn English to brighten their prospects, and the English they want to learn is American English.
There is no guarantee that this will be the last chapter in the story. Someday, it will be someone else's English that defines what is "proper usage." In the meantime, let's be clear what "proper usage" is. When we lived in a poor neighborhood of Terre Haute years ago, I used to correct my daughter when she spoke as her playmates did. When she protested, I told her that I didn't care how she spoke when she was out playing, but she had to remember that we were only temporary residents of that place. She was going to live among other people who spoke in other ways. If she wanted to be given full credit for her ideas, make friends wherever she went, etc., then she needed to be in command of a more respectable kind of language. Her little friends, I was sad to say, would find some day that certain opportunities would be closed off to them because they didn't present themselves by how they spoke and wrote as well as they might want to. In effect, Anna had to learn to be multi-dialectal.
Northumbrian > West Saxon > West Midlands > Chaucerian > Oxonian > American. I don't look down on non-standard dialects. The story of English is the story of non-standard dialects replacing what everyone thought would define "proper English" for ever. But whatever is currently the standard makes it possible for everyone, all over the world, to communicate in English at their best. Those who speak and write in that standard dialect have more opportunities, are given greater credit for their ideas. This is why we teach it in school. On the playground or the internet, you can use pure LOLspeak.