You are absolutely right about what you wrote. However, you are completely disregarding that the shaft driven motorcycles need to differentiate torque applied to two axles: one on the wheel and the other on the swingarm. Also, toque and power are the good sides of the deal, but torsional vibrations, inertial vibrations, driveline virbations and velocity fluctuations are the bad sides of the deal concerning motorcycle drive shafts. It seems many here have some odd idea these will not affect efficiency. I like your optimism, but reality does not work that way.Motorcycle shaft drives don't need to differentiate rotational speed between 2 wheels, since there's only one drive wheel. ( ignoring fringe off road 2X2 bikes that don't use shaft drives. )
There are various ways to dampen or redirect the torque trying to raise & lower the rear wheel on the swing arm. None matter to us here. They don't apply to airplanes and aren't differentials.
Finally, who cares if you can't find a name for "the thing which is not a differential, but many call it a differential"? I'll just keep on calling it a differential beacuse millions of people call it that name, unless you have another better standard or official name for the freaking thing. If you got nothing, I can't call it nothing. Now, does this "thing which is not a differential" lose efficiency big time? Yet, it does. Much more than straight bevel gears, which are noisier and less strong, but apparently do not lose so much efficiency.