I'm not really seeing how something signed in the 70s is going to prevent France from repairing a devastated building, that is French to the core and located in the French capital, however they see fit. If they come out and say it makes sense to use non-flammable materials for the reconstruction (which would make sense), who in the world is going to stop them exactly? Notre Dame was already damaged by the fire (which this treaty couldn't prevent), and now it's about repairing it.