Car brakes have dual hydraulic circuits for backup. You would think such an important component in a aircraft would have a backup and also a fail-safe mode.
So taking the car analogy one step further, what do you do when the brake pedal breaks?
Am sure it's been thought of but a fail-safe mode for pressurisation is a bit problematical - sometimes you want more air in the cabin and sometimes less - overpressure can be a hazard too (ok there are pressure relief valves but these could fail also).
There are different ways of controlling the outflow valve but as has been stated anything mechanical can fail. This is why crews are trained to deal with this eventuality.
Maybe air travel has become so reliable that passengers have losted touch with reality. Decades ago a/c were far less reliable and "emergencies" due to mechanical failures much more common. When's the last time you heard of an airliner suffering structural failure? Recall an article written by a pilot who had entered aviation in the days of Constellations and StratoCruisers. In those days it was almost an everyday occurence to shut an engine down on an Atlantic crossing due loss of oil etc. Said pilot converted to the B 707 and after 8 years realised he had not had one engine failure since he's been on the type.
I am sure if anyone could design a failure proof aircraft they would but this is a dream which is doubtless impossible.