Yes, of course they are though, I suppose that depends on how you define a 'dialect':
Here's one:
"Everyone speaks a particular dialect, i.e. a particular type of English distinguished by its vocabulary and its grammar. Different parts of the world and different groups of people speak different dialects: for example, Australians may say arvo while others say afternoon, and a London Cockney may say I done it while most other people say I did it. A dialect is not the same thing as an accent, which is the way a person pronounces words."
To say they're not would be to deny the existence of for example the dialects of the southern states with those of the NE seaboard. Unless of course you hold that the English of New Orleans and that of say Boston are in fact separate languages which would be patently absurd.
I agree that's more targeted at social and educational sub groups than linguistic variance for it's own sake as you suggest. I do agree that the wrtings of academia are broadly indistingushable on each side of the pond, down the the 'ize' suffix. But I would argue that academic writing is sufficiently removed from the day to day language to be considered a dialect in its own right. You would no doubt disagree.
Educated English, written in grammatically correct form, is essentially indistinguishable, whether written in Boston, New York, Atlanta, Chicago, Los Angeles, or London. The few distinguishing features are mainly some minor spelling changes (artificially introduced by Webster, and so not evidence of any sort of drift), some recent neologisms (hard to avoid over two centuries), a tendency in London to treat the names of corporations as plurals ("Harrod's have elephants in stock" vs. "Harrod's has elephants in stock"), and a very minor difference in abbreviating some sentences ("I should mail in my taxes; really, I should do" vs. "I should mail in my taxes; really, I should.") This isn't "academic" writing, but the writing one would routinely see in the Times (either one).This is the rationale for my claim that American English and English English are not really different dialects. The differences are too trivial. However, independent of that, American English or English English as spoken by fishmongers or cabbies or rappers might or might not be considered dialects.
The argument that English, or rather Middle English developed as a creole or Pidgin from Middle English is one I've head before. Old English as you will know is itself only parts of old Frisian, Saxon and Norse with Latin and other influences anyway and itself had mutually unintelligible dialects.
I've yet to see the theory developed, but it seems obvious enough to me. The textbooks usually lump Modern English in with the Germanic languages, particularly close to Frisian. I'm reluctant to agree with that classification, for structural reasons. Old English/Anglo-Saxon, Frisian, Old Norse, Faroese, Dutch, High German, Low German, and Icelandic are Germanic languages, no argument there. But Modern English is a hash of vocabulary from Old English, Old Norse, Frisian, Brythonic and Goidelic languages, Church Latin, Norman French (itself with a hefy Old Norse component), and a bit of Pictish, all strung together with a grammar entirely different from any of them. All of which says, to me, "pidgin", leading of course to a creole.
I'm not sure it's excitment, so much as appreciation of what is a great language. Your language suggests that it's somehow less worthy for its origins (as you believe them to be)?
I suggested no such thing.
I made a hint that squawking over which bit of dirt is home to the more original pidgin is a bit silly. No pidgin is "original", by the very nature of pidgins.