Population replacement seems to had been significant but in no way total, the ancestry of modern the population of eastern England (which was conquered/settled

Population replacement seems to had been significant but in no way total, the ancestry of modern the population of eastern England (which was conquered/settled first) is around 40% Anglo-Saxon/Continental Germanic (however the influx of Germans continued well after the 5th century and we can't really genetically distinguish the Saxons/other Germans and the Vikings/Norse whose came centuries later). Of course we don't really know what happened to the local people, many might have fled west or to Armorica, there are some hypothesis that the native Briton population was more heavily hit by the plague and/or that south east England was already significantly depopulated during the 4th century, many were undoubtedly murdered however it seems that the majority (or at least a number exceeding that of the invading Saxons) survived and were integrated into the new Saxon society one way or the other. There is evidence that there was at least some degree of peaceful coexistence between Saxons and local British at least in the early periods of settlement. There are burial sites where both genetically British and German (or mixed) people were buried together, with identical grave goods and seemingly with no distinction. This would indicate that at least some local people adopted Saxon customs and language and were accepted as equal members of their society. However this is not exactly conclusive, there are also many segregated burial sites so the type of interaction between the local Britons and the German invaders/settlers varied by area and time period, it took them well over 200 years to almost fully conquer the territory of modern England. Gildas and St Patrick were members of the Romano-British landowning elite which was displaced by the Saxons, I'm trying to dispute their authenticity but their experience might not have been representative of the majority of local people (who were never fully romanized or integrated into the roman society). It would not be completely surprising if local Britons decided that they would be better off by 'switching sides' and adopting Saxon/German culture. Of course we don't know how widespread and voluntary the process could had been, however according to genetic evidence it must have occurred to some extent.