That is a blessing. You can re-build a great cathedral - though it will never represent the same historic accomplishment; but you can’t re-create the artworks.
Even if it was an act of terror, Christians should remember that not one life was taken, and it was just a building. We should react like Whoville did on the morning of Christmas - by singing praises to God the Most High, who created the heavens and the earth and all of us on it.
“That is a blessing. You can re-build a great cathedral - though it will never represent the same historic accomplishment; but you can’t re-create the artworks.”
You rebuild the cathedral. This way, 800 years from now people will be talking about the fire that destroyed it in 2019 and how all the art was saved by the courageous members of the church. It becomes part of the cathedrals history.
I certainly don't intend to minimize this tragedy, but the spire that burned and collapsed was apparently itself a rebuild from the 19th century, as opposed to the original 13th century structure.
The most unfortunate loss from a historical standpoint seems to be the wood supporting the vault - which had been among the few original works of Medieval carpentry that survived in original form for 800+ years.