Who was talking about using bike gears? But also, you should abandon derailleurs at any speed unless you’re racing the tour de france. The only advantage they have over internal hub gears is weight. And hub gears can be easily made strong enough to handle that sort of torque.
But you wouldn’t necessarily need gears At All for something throttle driven. Electric motors have more than enough range of RPM on a single speed gearing system to get to 45mph. E-bikes only have gears for the human component, not the motor.