An automatic guidance system in a fixed-wing aircraft is given a route with waypoints, and autonomously navigates between them. It needs to know things about horizons, Great Circles, collision-avoiding altitude change plans, least-time routing, reachability of nearby airports given available fuel weight, etc. There's no real way to implement this with anything less than a CPU with a full (realtime) OS and a software stack on it, full of complex pre-baked databases, with multiple inter-related components from different vendors in the mix. It's frail. Of course it needs a backup.

The kind of system I'm talking about here, on the other hand, is essentially a physical model of the three available controls and how they interact to move the aircraft (and thus predict how the aircraft will be moved); combined with actuator-sensors, accelerometers, and gyroscopes to read off how the aircraft is being affected by those controls (and thus correct the prediction)—enabling you to translate linear movement of a flight-stick to linear movement of the helicopter. It's a dynamic torque model feeding a control system, with a user-supplied reference point. You can make an ASIC that does that and put it through fuzz-testing for literally every potential input combination it'll ever receive.

Yes, said ASIC might physically fail (even after you've done an extensive burn-in test)—but, given the tolerances we have in making ruggedized solid-state electronics these days, vs. the tolerances we have in making mechanical systems, it'd be the least likely part in the control chain to fail.

It is obvious you have never actually flown a real plane, and you have no familiarity with actual aircraft avionics. There are so many errors in what you said above that I don't have the time to correct them all.

It is true that state-of-the-art navigation systems operate the way you described. But airplanes have been flying for over 100 years, and there have been autopilots for nearly all of them (the earliest autopilot dates back to 1912). The vast majority of airplanes flying today do not have state-of-the-art systems.

You are just picking random descriptions of autopilot systems.

Helicopters can also have advanced and full integrated routing systems that will complete an entire trip from A to B, just as fixed-wings can have a simplistic input-to-output linked autopilot that just aims to keep the plane level and with the same heading.

So yes, an advanced system is more complicated than a simple system, but both systems can fail. And since that is inevitable, it's usually better to have a complex system with redundancy that will tolerate failure, than a simple system with no backup that will lead to catastrophe.

it is not the electronic chips that cause an autopilot to fail (in most cases). the primary failure mode is 'something else', meaning a failure in the mechanical equipment connected to the environment.