That’s actually a common historical misunderstanding. Anglo-Saxons didn’t eradicate or exile the natives, they simply replaced their ruling class. And even then, Brythonic nobility were allowed to rule under them provided they swore fealty and adopted their customs. A lot of people don’t realize that ethnic cleansing is very resource intensive, and why would you waste perfectly good subjects/peasants (provided you aren’t brain-rotted by early modern nationalism or race theory)? It’s not like they had enough settlers to feasibly do it anyway.

This was pretty much the norm of the time for the migrations of various Germanic tribes west except the Anglo-Saxons were actually successful in imposing their languages on the natives rather than adopting the locals’ (as had happened with the Visigoths, Franks and Vandals), probably because Roman Britain was one of the less developed and populated provinces. It’s not even a loose theory: historical records show “English” lords with mixed or completely Romano-British names/titles and DNA testing shows most Englishmen are descended to the Stone Age people who came before the Celtic migrations.

I think this is important to point out because it’s the exact kind of misunderstanding Zionists exploit by claiming Arabs wiped out the entirety of the Middle East and North Africa, so Palestinians are ackchully foreign colonizers themselves. It’s the same story there: the language of Arab Muslims, either by sword or by conversion, was spread throughout pre-existing populations though not entirely (see Maghreb languages).