There was talk about restoring the Beckhampton Avenue, but when asked about this, one of the points raised was: "Which Beckhampton Avenue do we re-build"? There was no one, defined version, its construction was fluid, and added to, until it fell into disrepair.
So maybe to think of a Cairn as "complete, and not to be changed, restored, damaged, added to, subtracted from" is missing the point. At what point was the Cairn finished? At what point is adding to it over? How big was it when it was finished? When the people who first started the mound had died? Did their children and other generations add to it? Do they only have the right to add stones or is it a tradition to carry on adding to it? Have people been adding to it constantly over the centuries?
We always like to think of monuments being "finished", but if history has told us anything, it's that these sites have been used, added to, depleted, restored, destroyed again, restored again, over and over.
It is all fluid, always constantly changing. Only this generation thinks about "preserving" the monuments, "Saving" the monuments. Perhaps inadvertantly, destroying (the essence of) the monuments.