> If you have an electric plane with two motors, a big APU-like turbine charging a battery and powering the motors, you might get away with a cheaper turbine, running in less extreme regimes, and still have a more fuel efficient plane r

This works for hybrid cars because the power demand when driving varies widely, as does the output speed. The 'hybrid' bit gets huge gains by always running the engine at optimal rpm+throttle, and using the battery to cover peaks and absorb troughs (regen).

A plane cruises for many hours at a fairly constant speed and throttle, which is designed to hit peak efficiency of all components. The 'hybrid' design therefore falls far behind on weight, efficiency and cost.