Aren't the mechanisms of Moore's Law (ie. smaller transistors that run at lower voltages) exactly the same things that make chips more susceptible to radiation? Once you compensate for that by including more redundancy, you may not have a cheaper chip.
Yes. More dense, lower power chips are more susceptible to radiation.
And anyway, what exactly does "redundancy" mean? If a rocket engine controller is triple redundant, how does that work? Are there three propellant valves in parallel, so each computer controls one-third of the thrust? Are they in series, so that failure of one computer disables the propulsion function? Is there a majority vote system, and is it electronic, electromechanical, or fluidic? Redundancy is not pixie dust that magically makes your system design better.
A sample Google interview question is to design the protocols to run a cluster of unreliable computers. Should there be a MIL-SPEC master computer? Should the cluster elect a master? Or several oligarch servers? Where does an outside agent submit a request, and what does it do if the request is not answered. Designing reliable systems is hard.