[foobarbecue]

Multiple rotors do decrease efficiency and this flight time somewhat, but nowhere near as much as switching from fuel to batteries. The mass energy density of batteries is less than 0.1x fuel. There are some gas-powered quadcopters out there which much longer fight times than the electric ones. Compare an electric CP RC heli to nitro and nitro has much longer fight time. Of course, a combustion engine is usually heavier than electric motors too, so there are diminishing returns from switching to fuel as your fuel or battery to engine mass ratio decreases. But, the main consideration in this regard for flying things is almost always: fossil fuel is way, way more energy dense than batteries in a mass sense. See e.g https://www.batterypowertips.com/comparing-ev-battery-and-fu...