Explain Like I'm Five

The Romans had an impact to a greater or lesser degree across the whole of the area that they controlled in Great Britain, including Cornwall and Wales, but the Brythonic (Celtic) culture seems to survive for most ordinary people throughout that time. It was really only the arrival of the Germanic peoples - the Angles and Saxons - that seemed to displace the Brythonic language and culture from much of the lands that they went on to occupy, which was largely the land that was easier to work in the majority of England, but not the more difficult land in the West and North - including Cornwall and Wales.

Around that time, there is evidence that some Brythonic speakers were moving into Wales - presumably from England - causing changes to the existing dialects there, also some Britons seem to have migrated to Brittany on the continent, and there was an outbreak of plague that affected much of the Roman lands and caused a population decline there - but less so among the Germanic people.

No matter which had more effect, it was the Germanic people and culture that displaced the existing one - not the Romans.