It's quite hard to challenge the orthodoxy in most fields, and language is no exception. This only seems fair - you need a lot of evidence to disprove an existing theory... or do you? I suppose that depends on what evidence there is for the existing orthodoxy.
The orthodoxy of language origin theories is often distinctly lacking in evidence, as in many cases it is tied into notions of ethnic origin, most of which are being proven wrong even as we speak.
A great many national ethnic origin myths are based on the idea of conquest as an annihilation of the existing population. For example, take the Anglo-Saxon invasion of England and south-east Scotland. Received wisdom is that the Norsemen started invading the territory roughly corresponding to modern Denmark, and started killing and displacing the locals, who fled across the North Sea and started killing and displacing the local Celtic Britons on the east coast of Great Britain.
That gives us the creation myth of England, that was built on hearsay.
I mean... did the Romans kill and displace the locals? No, they ruled over them. We were asked to believe they were the exception.
But did the Greeks kill and displace the locals? No, they ruled over them. Another exception, perhaps? The Moguls in the Indian subcontinent? The Mongols?
And more recently the Ottoman Empire? Or indeed the French or British Empires?
The fact of the matter is that the overwhelming majority of well-documented "invasions" in history have simply involved the installation of a new elite over the existing population. The genocide perpetrated in the Americas and Australia is actually pretty unique in history.
So why are we expected to assume that all the poorly-documented invasions are so different from the documented ones? Why shouldn't the Anglo-Saxon invasion be more like the Romans?
Well, thanks to DNA, that old orthodoxy has been turned on its head. Genetic testing suggests that there's a heck of a lot of Celtic ancestry where the old kill-them-all theory of historical invasion would have left us with only Anglo-Saxon blood. On top of this, in the old Anglo-Saxon homeland around Denmark, they still have predominantly Anglo-Saxon DNA, where we were supposed to believe that the Norsemen had killed or exiled all the Anglo-Saxons.
But we shouldn't have needed DNA evidence. We know that the Anglo-Saxon kings of England claimed descent from King Arthur, a figure not from Anglo-Saxon mythology, but from British Celtic mythology. This alone should have proven conclusively that a huge portion of the English population was drawn from Celtic stock. Not to mention that the Danes have never looked all that similar to the Norwegians and Swedish. The Norse Vikings that invaded the west of Scotland were towering red-heads, while the Danish Vikings that invaded Northumberland were normal height and had dark hair.
This naturally has direct consequences in the origin myths of languages.
The best documented case of language birth brings us once again back to the Romans.
We know a heck of a lot about Classical Latin, but we know very little about the colloquial speech of the common man in ancient Rome. We know it was different, but we don't know how much. We also know that when they went out and conquered other nations, the locals tried to learn Latin, but did so imperfectly. We know that these imperfect Latins eventually developed into the Romance language family we know today (French, Italian, Spanish etc). Crucially, while we know very little about the early stages of the Romance languages, this is because Latin remained the language of the elite, so all literature was in Latin.
This means that the single best-attested example of language evolution revolves around an elite language hiding changes in the common language.
We can compare this to creole languages. Even today, many creole-speaking countries are run by elites that prefer to speak in French or English, and resist efforts to raise the profile of the local creole language.
All our most reliable data on language birth comes from languages where a large population have imperfectly learned the language of the elite, and where the elite language and the common language have existed in parallel for a significant period of time. It stands to reason that this should be our default assumption for all languages.
And yet our standard model of language development is still based on time periods: Old followed by Middle followed by Modern.
Given all that, it came as no surprise to me that someone recently published a paper proposing that a lot of so-called "Middle English" was actually contemporaneous with "Old English", and that the loss of grammatical complexity wasn't due to the Norman invasion (the Normans were a tiny minority elite ruling over a huge population) but rather the result of the Britons imperfectly learning the Anglo-Saxon of their rulers. The reason he gave for the apparent abrupt change was quite simple: the Anglo-Saxon elite were disenfranchised and Norman French became the language of literature. Middle English literature was not written by the descendants of the old Anglo-Saxon elite, but by the descendants of the peasantry, or people of Norman descent who learned it from the peasantry.
Shouldn't that have been our default assumption all along...?