That's absolutely how it works in engineering. You build fail-safe systems when you can, if safe-life systems where you must.

There is no redundancy in the structural integrity of an aircraft wing. Once it falls off, everybody dies.

Similarly, there are little redundancy margins in a spacecraft propulsion system. You will plan for a thruster malfunction, but if you loose your entire control system in flight, or if you develop 5 different leaks in flight, than it's safe to say you have a failing system. At no point in the design phase were any of those failure modes deemed "acceptable".