From what I understand of the orthodox stance on the Anglo-Saxon invasion, its wasn't an invasion where, like the Norman's later, the ruling classes were made of of the new arrivals. It was more of a colonisation - with large numbers of Angles/Saxons shipping over to Britain and taking over land.
If this was the case, and meant that the native population were either driven off their lands or were assimilated into the Anglo-Saxon populace, wouldn't this explain why the language changed?
Also, wouldn't it explain why in the some more remote parts of Britain different languages (Gaelic/Welsh/Cornish) survived?
To give a later example of what I mean here - the original natives of North America didn't speak English (Or French for that matter) until the land was colonised by Europeans - within a relatively short space of time the native language(s) were replaced by European languages At this point, although there are people left who still speak the original native languages as time passes, older generations die off and younger generations assimilate more into the dominant culture the native languages may disappear completely. Could this have been the case in Anglo-Saxon Britain?
Apologies if this is covered in the book - only ordered it yesterday!!!