"The weight of the water and the intensity of the drop at low altitude could indeed weaken the structure of Notre-Dame and result in collateral damage to the buildings in the vicinity."

So, yeah, I'm gonna trust the experts on this one, not random armchair Internet quarterbacking.

The facts stand, as have been reiterated in the thread above:

- Firefighting airplanes can effectively spray in a way that doesn't harm trees (by releasing the 12,000+ gallons over a very large area)

- Buildings are fine in extended substantial downpours (which is significantly less water per second than an air-tanker dump)

To your thought's end:

- Spraying a building with a plane in such a configuration would not collapse it

Yes, it's possible to spray the cathedral with water in such a way that it will not collapse it, but that configuration is that very little water gets on the cathedral, and is instead spread over pretty much the entire island in the Seine.

At this point in time, however, it likely doesn't matter. Since most of the building has collapsed already, due to the fire. Like we said it would.