Two pilots are required as crew, and it is impossible to supply identical pilot one for one. In handover, the new pilot is a different solution, by definition, so redundancy in the case of pilots is not an issue, in fact, the "back up" is a fresh resource with different solution making potentials by definition.
Hm, I think that is only partially true. Rigid procedures & training aim to make pilot behaviour both predictable and, consequently, identical.
In the case of AF447, we seem to see just that - but in a failure mode. Three pilots did not understand what was happening, and failed to fly the plane.
That leads me to another thing in the report that sends shivers down my spine: The stall warning was sounded more than 70 times during those fatal minutes, and there was no acknowledgement by any pilot of that, neither through action, nor through speech. It has been speculated that the pilots did not perceive the stall warning, maybe due to sensory overload. To me, it seems much more likely
that they did not believe the stall warning. And if that is the case, simultaneously with three pilots, it tells a chilling story of the trust these professionals had in their technology, their aircraft, their instruments. Apparently zero. Wonder why??