Languages do seem to be a rather poor guide to the mass movement of peoples: however the evidence that a lot of Britons stayed on under the Anglo-Saxons does not contradict the equally strong evidence that quite a lot did not.

Moving backwards in time, movement throughout the prehistoric Atlantic culture can have spread genes around the whole littoral: in which case we would expect to see such evidence in Ireland, Scotland, Cornwall and Brittany too. If not, then there is an argument for a distinct origin for the Welsh that precedes the traditional Celtic fringe view. It does not necessarily contradict it.

If these genetic markers are not found in Cornwall and Brittany, then this strongly suggests that at some stage in their history, perhaps pre-Roman Britain or (more likely?) even further back, there was a separate people in Wales, leading to the interesting question - why there? We know that Cornwall was trading tin - what else was going on?

As for "whole cloth" arguments - The DAN evidence suggests that this is simply not true, that we are indeed made up from differing strands. Mongrel indeed, but mongrels are rarely identical. Such a commonality may be desirable socially, but I fail to see much evidence of it actually being present. To the contrary, we seem to rejoice in our differences. (As an example, upon moving into Lancashire I was surprised to find that the rivalry between Yorkshire and Lancashire, a pantomime joke to the rest of the country, is still all too real on the ground.) All the more reason to better understand our origins.