Helicopters are not fixed pitch... your run-of-the-mill hobby multi-rotor is, but that's due to the reasons mentioned above in my previous posts (eg. simplicity at this small scale).
> Safety would be provided by redundancy
Unless you suffer a total electric failure. Or even just a failure on one side of the aircraft (or even just one motor, in a quad setup). Being able to mechanically work these systems is fairly important, since it's less likely both mechanical and electrical systems fail simultaneously.
> In a hybrid system, you anyway decouple the propeller and turbine with the electric system
Unless your electric system is the failure.
> Your car gearbox operates at much lower speeds (because it's not attached to a turbine)
Even some model multi-rotors use gearboxes. You have a motor, spinning either slower or faster than the desired RPM for the rotors... a gearbox solves that difference, and provides a mechanical means of fully decoupling from the motor should the need arise.
> Helicopter powertrains require a lot of maintenance
This is true, but this is not really an issue... and nobody really is looking to "solve" this non-problem. Aircraft require a lot of maintenance and inspections.
With collective pitch, it becomes interesting. Does the motor help or hinder autorotation?
In a hybrid system, you anyway decouple the propeller and turbine with the electric system.
Your car gearbox operates at much lower speeds (because it's not attached to a turbine) and has a lot less weight constraints. Helicopter powertrains require a lot of maintenance.
The stack exchange link talks about jetliners while we're talking about copters.