I've always thought this kind of analysis doesn't look closely enough at the impact of non-English-native speakers on the dominant accent of American English.

I've always thought this kind of analysis doesn't look closely enough at the impact of non-English-native speakers on the dominant accent of American English. In other words, the vast majority of Americans are not from English-speaking countries, and the immigrants learned English when they arrived. Thus the English spoken in the key port/immigrant destinations, plus the impact of all those immigrants' accents, should massively impact modern American English. One theory I've come to is based on the fact that modern Dutch-speakers of English tend to have a very American (rather than Britsh) style accent. I propose that early New York, being Dutch and the most important immigrant gateway later on, probably had a bizarre Duthified English which had a scaled impact on generations of immigrants who came through New York. Similar stories are probably found elsewhere.