Well, actually, according to some fire experts, Trump's proposal was not feasible. I agree with @Weird Tolkienish Figure Trump is once again nattering about a subject he knows nothing about.
His basic idea of getting a lot of water on the fire fast isn't so bad, really.
Problem is, that the stone might not handle either the thermal shock of massive amounts of cold water dumped on it, nor the weight.
If you look at the aerial shots of the fire, it appears the wooden roof portion is burning and collapsed onto the stone interior roof/ceiling. That makes access difficult, but dumping water on it from above in mass quantities could do even more devastating damage. From the interior shots, one portion of the ceiling collapsed, the darker area in the aerial shots, but otherwise, getting water on the burning roof materials has to be done from the aerial towers and monitor nozzles that can be deployed to put water over the walls, and carefully, to keep from overloading the structure with water weight (at 8.35 pounds per gallon).