They don't even need the AI navigation problems of delivery or military drones to be solved, just to be remote piloted along a given route. But all that solves

They don't even need the AI navigation problems of delivery or military drones to be solved, just to be remote piloted along a given route. But all that solves is removing the risk to the human pilot and their weight - possibly being able to get closer to the fire. We still need to move volume of water to the target, and that means getting a lot of flying vehicles to the right location and time. There's been plenty of big area fires, but this is as bad as it gets in terms of population and assets exposed, so we need a lot of vehicles delivering water to a lot of locations simultaneously. Being prepared for this scale of operation means either having specialty firefighting planes and drones in reserve, or having other vehicles - general purpose drones or cropdusters or, heck, a Globemaster with a big ballon full of water in its belly - that can be repurposed and sent to this emergency from all over. Airships would be nice for the volume, but they aren't exactly friendly to the turbulent conditions near a fire.