What is the business case for drone delivery with regard to energy efficiency and total cost of ownership and fundamentally to deliver? Except for high value, l

What is the business case for drone delivery with regard to energy efficiency and total cost of ownership and fundamentally to deliver? Except for high value, low-weight items, how do you see competition in the coming years in the face of fleets of autonomous terrestrial vehicles that don't have to lift their payloads into the air? I know Amazon has said previously most of their items "can" be delivered by drone, but the question remains should and will they, considering the high cost of current battery technology. Will it ever really be financially viable to lift items into the air in an individual manner when you can get them to their destination far more efficiently via the ground? It seems to me the case is for elimination of the human component in delivery. If drone delivery is really so important, why aren't the major shipping companies working on it and scraping their existing air freight and hub and spoke distribution systems?