You could get around fine with 2, 4, 6, it's just that it would have very slow and less efficient acceleration (accelerating for longer at low RPM and very high torque is overall less efficient than accelerating for a shorter period of time at a higher RPM, given identical start and end speeds).

And that's exactly the point I made. You need many gears for an ICE because being able to reach a given speed in a given gear says very little on how efficient that is, therefore you need different gears for accelerating versus holding a given speed to get acceptable efficiency.