Talking About Current Events: Would 'flying water tankers' have been able to successfully extinguish the Notre Dame fire?

Would "flying water tankers" have been able to successfully extinguish the Notre Dame fire?

No. Firefighting “bombers” would not have been able to successfully extinguish the fire that seriously damaged the Cathedral of Notre Dame. On the contrary, they would have just completed the destruction of the ancient structure that the flames started.
To wit: Call them air tankers, water bombers, or aerial firefighters, fixed-wing aircraft that drop either water or flame retardant are fine for helping douse or at least diminish wildfires in remote, unpopulated areas such as forests or national parks.
In heavily urbanized areas, such as Metro Paris in France, water bombers are useless.
I mean, okay. The planes will douse the fire, sure. But they’ll also cause a lot of destruction, injuries, and even loss of lives.
Air tankers are not precision instruments. If we were to make an analogy, using a water bomber on Notre Dame is like using a sledgehammer to swat a mosquito.
An Evergreen 747 Air Tanker has a carry-drop capacity of 19,600 gallons of water/fire retardant. A P-3 Orion (a civilian version of the venerable U.S. Navy ASW patrol aircraft) has a capacity of 3,000 gallons.
Okay. Think about how heavy one gallon of milk is when you first pick it up, full and unopened, at your local supermarket. Light as a feather it is not, right?
Imagine, then, the impact of “just” 3,000 gallons of water or any non-flammable liquid on anything, adding to it the forces of inertia and gravity.
It wouldn’t be anything like your average rainstorm or cold indoor shower. It would feel as though something wet but heavy and painful hit you. And, unless you’re incredibly lucky, it will be the last thing you feel.

In addition, aerial firefighting works best in situations where you can dump tons of water on a fire without having to worry about collateral damage. You can't simply "bomb" a target, even one the size of a Gothic cathedral, with water or flame retardant and not damage or destroy buildings, vehicles, and infrastructure in the adjacent area. 
Dropping tons of water on a building on fire, especially one that lies in the center of a heavily populated area, is not a good idea.
Unless, of course, you want to destroy it.
This notion is one of the reasons why Donald Trump should be seen and not heard.