What is important and is missing in most of these discussions is evidence. Mick himself says his book is one of assertions, and it clearly is not evidence based.
That's true of much of this discussion -- for instance, Ishmael can write about the Anglo-Saxon invasion that "We have nothing to indicate that the Saxon invasion was any different from the Roman, Danish or Norman conquests (as three specifically British examples) which effected no such change in the spoken language."
All these invasions were different in important ways -- as was the context of these invasions. The actual evidence is abundant and makes that differences abundantly clear.
By the way, a fundamental assumption of European archaeology was that the monuments of prehistoric Western Europe were built by assistance from the East. Then Colin Renfrew wrote Before Civilization, and that fundamental assumption was not just questioned but overturned -- in an academic book. I'm sure that that example is not unique.