Last paragraph first - you have missed off an additional possibility and the favourite at present, which is the roots of english came to Britain with a wave of settlers from mainland europe around 400 AD.
What exactly do you understand by de-urbanisation? I'd appreciate an explanation, it's not a term I've encountered before.
Replacing languages in 400 years? Visigothic current in Spain and France, Ostrogothic current in northern Italy and provence, both from around 500 and extinct by 900.
Welsh - very nearly rendered extinct between 1700 and 1900. Ditto Scots Gaelic.
Kernow, Manx - almost extinct and only preserved by extreme effort among a minute number of speakers.
Genetic evidence (including the link you supplied) doesn't actually prove that the Anglo Saxon settlement of Britain was non-violent - it may or may not have been, and is unlikely to have been one event of one type.
I'm not relying on any Victorian hypotheses. I'm relying on mainstream empirical research done by people who have spent their lives in this field of study at a high level. With respect, the line of argument you are presenting seems not to have extensive backing in the form of evidence.



