I'll double down and continue to contend they are abject morons.
I know the preventive costs would not have been trivial but doing something would have been frick-all trivial compared to what they have to do now, right?
Water supply? The cathedral sits (or perhaps sat) on a fricken island. We put a man on the moon a half century ago. Some genius could have figured this out. Don't like water? For one of the world's greatest cultural treasures, invest in some halon.
So say you allow that the place was a fricken tinderbox and that could not possibly be remediated. Failing all of the above, if you have some fricken Pierre on the roof with a torch, you'd better have 10 more Pierres with fire extinguishers above, below and around him watching what was going on.
Some big shot over there was already quoted as saying they ruled out foul play. I don't see how they could possibly have done that so quickly, unless some frog admitted he tossed a cigarette butt in the attic after he finished his dinner snails.
France is rantingly, proudly and stupidly secular. Part of me wonders if this is God's way of saying, "Enough is enough. If you ever want me back drop me a line, but until then I am out of here."