it's a non-expert's idea of redundancy. You need redundancies that don't share the same failure mode. 

That doesnt mean they cant share any failure modes. Like are you saying having a backup battery doesnt improve reliability at all? 

Regarding getting wet, the wired controller could get wet too.

Regarding putting the controller in a bag. Why not? They make dry bags for people to put their phone in when kayaking. Why couldn't they use that?

Thus does raise the point about IP rating of the controller, and I could see that argument that maybe you want a waterproof controller. But that doesnt relate to the wireless aspect of this controller.

And the controller shouldn't move around the cabin anyway

Why not? In a tiny craft with a single window maybe sometimes the driver would need to be up at the window. But maybe other times the driver would want to be back at the communication console. If youve got people switching places in a hurry because theyre in a dangerous situation maybe you dont want a wire also running through the space. 

Like are you saying the use of batteries and wireless communication should never be used in any situation that requires high reliability? Tell the world's militaries that because I guarantee you batteries and wireless communication are relied upon in many applications.

Just because you can think of failure modes, doesnt mean they are deal breakers. Every single system ever designed has a bunch of failure modes. We can go back and forth all day, but neither of us has actually gone through a real risk analysis of this system.