> What he fails to take into account is the fact that the wide availability of audiovisual distribution of language has a conservational effect that we've not really seen before. So for instance, while there is a large variety of English dialects in the US, "Broadcast English" and "General American" are quite stable and broadly available.
Is this true? I don’t think it is. Go watch or listen to some audiovisual records from not even a century ago, like a classic Hollywood film or one of the old WWII training films on YouTube for instance. If you listen for it, you will hear a dialectical drift.