It's a great solution for reducing mechanical complexity in mini/micro sized UAVs, though.
The added cost of batteries at even today's prices could be worth it for the decreased mechanical complexity of the rest.
If all we need to do is change the torque of an electric motor, that can be done essentially instantaneously even at high power levels.
Power storage may continue to improve, but it will take a long time before you hit the energy density of fossil fuels; the difference between batteries and fossil fuels is something like a factor of 10 (actually greater than that, but electric motors are more efficient so let's say 10 for the sake of argument), and it's improving much more slowly than Moore's Law, doubling maybe once every 10 years. That means if those trends continue (and there's no guarantee they will), you're looking at 30-40 years before the energy density allows electric helicopters to be competitive with fossil fuel powered.
For cars, energy density isn't quite as important, as the weight of the car is not the dominant factor in its efficiency (it does have an effect, but the aerodynamics, engine efficiency, transmission efficiency, tires, etc. all make a big difference too). But for a helicopter, every pound you add to the batteries is another pound you have to lift, so energy density of your power source is quite important.
So, if trends hold on battery technologies, it will come about eventually; but I would put money more on the decades timeline than the years timeline.
This is a material science question. A material may exist that has the right combination of flex, strength and lets also not forget durability. I am a mechanical engineer and crack propagation and cycle fatigue would be a major concern for a joint like that at large loads. There are all kinds of amazing tricks material scientists know how to play to combat these types of problem.
I think this could be scaled up if material to make those flexure joints exist.
With a regenerative approach, you'd probably need very little net electric power.
