Many people think that medieval art has been lost forever. Guess what? Wrong!
In the 18th century during a great renovation most of the medieval windows and interior was destroyed, on purpose. People thought that it is just way too dark in the church, so most windows were replaced with normal, transparent glass back then. The rest was done during the revolutions of 1794 and 1830.
So this means that the vast majority of the interior and fassade as well was made in the 19th century due to this. So this is not why Notre Dame is important; it is important, because the principles of Saint Denis were here developed much, much further, and it became a blue print so to speak for generations of following gothical cathedrals.
By the way the little tower on top of the roof which collapsed does not belong to the original plan; it was build in the 19th century. It is an element absolutely uncommon in French gothic. Now the question is, since many of the additions of the 19th century has been damaged or destroyed which did not belong to the original building - are they going to rebuild it, or just stick with the original plan? And which state of the cathedral are they going to rebuild - 1300, 1500, 1850? Tough decisions.
The roof itself is quite small, and there's tons of experience in Europe on how to rebuild such things - because it happens to churches now and then.