I didn't say it was impossible that a strain or two of the DNA might not have survived.
But even today family branches die out all the time from natural causes.
Considering the uncertain nature of mortality at the time, where whole populations died out in the 1st-3rd Centuries from disease, infant mortality, warfare and violence, the likelihood of uncovering a single line of DNA from only 500 subjects that lasted over a 1,500 year period seems rather remote.
yeah i would assume some genes are still mixed in but the y-chros. are likely long absorbed. Unless they were importing women to border forts, no foreign matrilinear dna at all. as you say, 80+ generations with multiple plagues and migrations (q-celts in some areas, saxon, viking, saxon again, then the 'harrying of the north' (did this extend to the wall?) it is amazing any celtic genes survive at all in that area
I wonder if there has been a study of people in the interior of wales (or cornwall) to see if any dna markers are similar to known italic dna, since they would have absorbed people from roman towns and cities, with civilian population including some romans.