Sourced, but, if this hypothesis is accurate, then why was the dialog in Dickens’ writing praised for so accurately presenting the nuances of the various regional dialects (a skill he developed while serving in some capacity as government reporter, where he could tell the region of a speaker by the accent)? Those sorts of nuances don’t develop in merely half a century.
I’m not saying that this is not possible, it is. But it is far more likely that the early colonists spoke the English they came here with and that the “America” dialect commenced when immigration began in earnest and Americans had been here for several generations. Thus, it is far more likely that we did, in fact, diverge from the English accent into an amalgamation of dialects from various immigrant accents.
Don’t forget, tho — usually the ‘colonies’ have kept some of the old word/ old wayss. It’s true in Spanish, anyway.
Boston certainly has the soft R sound. Back in revolutionary times almost half the population was German speaking, what influence did this have on our speech? I have been researching a book on the Revolutionary and post revolutionary period. There Scots immigrants were described as having a thick Scottish accent. My source book was printed in 1850 based on a 40 year collection of anecdotes.