Every source I can find, not nearly or most but every, states that North American English retains the older forms of the language.
As you have stated, isolated groups tend to preserve their speech forms more than those in dynamic trade centers. Estuary English certainly fits that dynamic profile.
All of North America received less than 20,000 immigrants per year until the mid 1800s. They came from two places, England and Africa, likely the larger part from Africa. (African American is a non-retroflex speech)
Most North American speech is rhotic, as English was in most places in the 17th century. Rhoticity was further supported by Hiberno-English and Scottish English as well as the fact most regions of England at this time also had rhotic accents. In most varieties of North American English, the sound corresponding to the letter r is a retroflex [?] or alveolar approximant [?] rather than a trill or a tap.
Many of the differences between American and Southern British are because of innovations in Southern British. The sound r was lost (except before vowels) somewhere just under 200 years ago in London. This change spread out and is now established all across England except the south-west and East Anglia, and is also true of the Southern Hemisphere countries colonized in the last 200 years. So American resembles Irish in being a rhotic accent (one having r everywhere) because they are both survivors of the original situation that 300 years ago prevailed everywhere in England.
Again, the American can't with the same vowel as can reflects the original English, and it's England that's innovated to the ah vowel. This can be approximately dated by noting that Australia etc also say kahnt; but similar changes in dance, plant happened in England after the settlement of Australia, where they have not been taken up: the can vowel in common between American and Australian dance is also what was said in London 200 years or so ago.
Considerable detail can be found in The Cambridge History of the English Language Volume 6: English in North America, a book I haven't seen, but there's a good thorough review of it at
http://cf.linguistlist.org/cfdocs/ne...cfm?SubID=4807
The same can be seen when you compare Brazil and Portugal…it was Portugal that changed.
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