Probably the greatest debate in the history of American English dialectology, so I thought I'd ask what yinz thought of it, or (even better) if there are any insiders here on academia of the English language, whether there's a general scholarly consensus on it.
My personal impression is that, although not as rapid or all-encompassing as some fear, there is a slow general trend toward homogenization. Most of the smaller dialects on the continent (NYC, Northeastern New England, Baltimore, Philadelphia, Appalachian, Western Pennsylvania) from what I've read are either markedly declining in distinctiveness and use among young people, or at least losing some distinctive features in favor of General American equivalents (e.g. the classic NYC split-/ae/ system giving way to the GenAm nasal system in most young New Yorkers) with few to no corresponding gains. The picture of Southern US English seems to be more ambivalent, with certain nonstandard features like /ar/ backing/rounding becoming more common, but I think it's safe to say this dialect isn't actively diverging. Even the Northern Cities Vowel Shift, widely held up as an example of continued dialectal divergence implied to be representative of the overall picture, has apparently been reversing in numerous Great Lakes cities, meaning that the only American English dialects that are unequivocally continuing to drift away from the norm are those of California, Miami, and (if it counts, since it also appears to be taking on more Californian features) the Pacific Northwest. In short, while not as extreme as in Britain, the situation seems fairly grim to me.
I don't WANT this to be the case - it's partially for this reason that over the last few months I've progressively rejected my native General American-like accent for a classic Chicago scheme - but I don't see how it can really be stopped with cross-national migration as vibrant as it is. Is there any evidence that I'm wrong about this whole thing? I would very much love to be.