yes, it seems clear to me that there is much less dialect variation in the US, outside very rural areas, than there was 30 years ago. Even then, the adults had much more variation than the kids.
I'm currently reading Steinbeck's "Travels with Charley in Search of America". He wrote it in 1960 and even then he remarks on how the US is quickly losing local dialects thanks to radio, television, and mass media.
I'm from N. GA which has a flowery dialect and all those expressions my grandparents used are dying. I think 1 or 2 more generations is all that's left.
There tends to be the most variation in the country where a language originated—-the “center of diversity”. Taiwan is the CoD for the Polynesian languages including Indonesian and native languages of Madagascar. (As per Guns Germs and Steel)
British people also sound more American than ever. Nearby, in Ireland, there are rich (suburban, I think?) Dubliners who have never left Europe that, with their normal accents, can briefly pass for Americans to Americans.
I spent 2 weeks in Iceland doing the ring road. We stopped for dinner in a tiny village in North Iceland. Our server genuinely sounded like an American. We asked if he had lived in America. He said he had never left his village, even to go to the capital. He just learned English from American media. His English was flawless. He could have said he spent most of his life in America and we would have believed him based on his accent alone.
Between schools teaching some form of standard pronunciation (in most countries, at least), and routine exposure to the same through mass media and intermingling in large cities, most languages seem to be on the trend towards less dialectal variety.
I can't possibly remember where but I remember hearing that there are actually more new dialects today. It seems like something that is in constant flux everywhere.
