You need to be able to decouple from the motor/engine for safety. Gearboxes provide this ability. One cannot autorotate if a motor has seized and locked the rotor in position, or has been damaged and pegs to full RPM. Also gearboxes can vary torque and RPM without having to adjust the engine/motor RPM/Power.

Also, varying power instantly isn't necessarily a desirable trait in a full-scale aircraft. You have rotors/propellers traveling close to, or exceeding the speed of sound. Instant acceleration would put a lot of forces never before seen on these components.

Usually in aircraft, the RPM of the rotor/propeller remains fairly constant, or within a small bounds. Typically you increase power to the engine/motor, and the gearbox keeps the rotor/propeller RPM's constant.

> unreliable gearbox

They're a lot more reliable than you seem willing to give credit for. Even in your run-of-the-mill car, the gearbox usually fails well after many other major components, especially with regular maintenance (where it may never fail over the lifetime of the vehicle). Aircraft are subject to very stringent maintenance and inspection schedules.

> The motors are there

Seems they aren't quite.[1]

[1] https://aviation.stackexchange.com/questions/26910/could-an-...

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.

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.