Historically most stonework was created by cannibalizing other stonework such as the famous Rosetta Stone being recovered while Napoleon’s army was attempting to strengthen their fortifications in Egypt.
If you had some sort of system, such as reducing the cost of a stone building when you put it down on top of a destroyed building that would encourage players to try and retake defensive positions. It would create a dilemma if say for instance a keep was destroyed The defender would be encouraged to retake his lost ground and put a new keep down on top of it. While the attacker would be encouraged to put their own keep down on top of the tenuous ground since it would be cheaper. Without our current system there’s no reason to do that so the defender and attacker will just put their keep back farther where it’s not at as much risk.