There is an important new history of the south west peninsula from prehistory through the Medieval period that takes on topic raised by your question: Craig Weatherhill, The Promontory People (2018). For this discussion, here is a map of the south west of Britain I have prepared for an article of mine, currently in final revisions.

First, Weatherhill does a great job taking apart the later assertion that King Athelstan’s tenth-century declaration that the Tamar River was the boundary separating English Devon from Cornwall included the subjugation of Cornwall, transforming it into a client state. Weatherhill demonstrates that none of the contemporary, primary sources indicate that Athelstan was able to claim control of the people to the west of the Tamar, the half of ancient Dumnonia that would stand alone as Cornwall. Weatherhill does demonstrate that a succession of Wessex monarchs were able to encroach with a stealth policy of land grands through the church, but this was not a military victory but rather one of patient diplomacy, and Athelstan's success was not complete. Cornwall was not subjugated.

Now, if you'll look at the map, you will see two "pinch points" that confronted invaders from the east. The most obvious one is the modern boundary that separates Cornwall and Devon, namely the Tamar River, which nearly bisects the peninsula, turning Cornwall into an island but for a small tag of land in the north.

The second pinch point is directly to the east of Devon. If you look directly north of the point where the eastern boundary of Devon meets to English Channel, you'll see that the land in "pinched in" by the Bristol Channel on the north. That "pinch" was much more dramatic than the map indicates for much of that northern land was marsh, making it largely impassable. This made the ancient kingdom of Dumnonia (the "mn" was pronounced with something like our "v" - the "v" of Devon), an easily defended place. The south west peninsula is long, remote, and easily defended at these two pinch points. Its rich tin reserves made it a powerful trading partner and a source of independent wealth.

The period when Wessex was most likely to make encroachments to the south west happened also to be the centuries when it was fighting for its life against Scandinavian incursions. Weatherhill also describes how the people of the south west formed alliances with Scandinavians to attack Wessex and to drive any encroachments back. So while, Wessex ambition was clearly to dominate the entire south west, they were never successful in that ambition.

One of the things that post-Roman Dumnonia was able to do was to establish a powerful trade network that included its reach across the channel to Brittany. This resulted in a "back-feed" colonization of the north east peninsula of what would become known as France, establishing the language that would be known as Cornish in that place. I'm less comfortable speaking to the survival of the Breton language in that place, but as a matter of overview, it is clear that the powerful who rose and fell during the post-Roman period as France attempted to put its feet on the ground, included a series of distractions that caused any would-be cultural imperialism to fail to reach across the Brittany peninsula, which was, like its British counterpart, remote.

I'm also not comfortable with comparing all of this with the situation in Cumbria, but there is no question that those people did not benefit from being remote and having the highly defensible real estate that blessed the Dumnonian peninsula: the "pinch" of Britain in the north actually makes much of Cumbria on the very road that must be taken to travel up and down Britain. Cumbria is practically "on the road" to everywhere, while the south west peninsula is on the road to nowhere.