the internal structure was wood, and that is why the walls around it are probably fragile with little support left in the areas that burned. As long as they don't get tipped over, the original stonework can likely be restored, given enough time. Bad day for a great historic structure.
I saw one report that said the first responders saw that the big open area was fucked, so they basically left the perimeter guys to shoot water but the entrance teams, because of the falling ceiling, just carried priceless artifacts out of the building as fast as they could. Good move, probably saved a lot of lives of firefighters.