While I really doubt drones will replace Air Tractors In their current role, (relatively tiny payload, limited duration), I bet that drones will be as common as tractors within a generation on cropping farms.
- satellite data is already widely used to measure crop health and pest detection. With the appropriate sensors, drones will do this with much greater resolution and in real time.
- as discussed, the appropriate sensors will allow targeted spraying of weeds, which would make payload less of an issue.
-on grazing properties, obviously fence checking, water troughs etc.
So in answer to the OP, I don’t think they’ll replace anything, but they will become another very useful tool.