The flight control systems of civil aircraft like the A320 has failback modes to handle hardware failures such as a failed angle-of-attack sensor
https://a320podcast.libsyn.com/flight-control-laws
The 737 MAX crashed because it didn't have fallback modes.
Engine Control Units in automobiles also have fallback modes. You shouldn't get stuck just because an oxygen sensor failed, even though that means the car will have trouble balancing clean emissions, performance and fuel efficiency.
WRONG: I believe the space shuttles started our with all the computers being LSI-11s. Presumably that was upgraded as the STS program continued!
Hmm, I looked it up and actually they were older: standard IBM avionics computers designed in the mid 1960s. They were all the same design and as far as I can tell from a little DDG searching, they were never upgraded.
I was so wrong I decided not to delete my mistaken observation.
System/4π derivatives were used for the target discriminating radar on the F-15 and quite a few other military applications.
Their very example -- airport notice boards -- is an example of someplace where fallback is needed. The thesis of the piece is that management of fallbacks is complicated and painful and thus increase the scope of failure, as you observed.
In other words: fallback is often but not always required, and if you can plan to avoid it it may be better for you, depending on your application.