Some monuments are safeguarded today, but not all. Fewer monuments were safeguarded in the millenia before we understood radiation. Your design relies on a cult

Some monuments are safeguarded today, but not all. Fewer monuments were safeguarded in the millenia before we understood radiation. Your design relies on a culture that values the remnants of other cultures to the point that they are sacred. There have been times and places where the people just wanted to re-purpose the nice building materials like the Mayan temple that was destroyed to create road gravel. Wars can break out that destroy delicate structures too, like the archaeological artifacts that were scooped up by the U.S. military to fill sand-bags during the Iraq war. Choosing huge, ugly, low-value materials for construction avoids those dangers.