Ah, but there’s the rub, you have qualified a time frame and thus have to say that there is a statistical probability of failure in a certain period of time... MTBF.
Even Gravity will eventually fail but not in a time frame we should be concerned with. An asteroid may hit, the Sun will Nova, the magnetic field will shift (it already has more than once) but will any of these things happen in a time frame important to us now? Probably not.
You said:
Gravity doesn’t fail. Would the system have ever failed? I don’t know, I guess, maybe if they ran it long enough so things were crumbling into dust. Certainly everything has that kind of endpoint failure. But for the practical lifetime of a system
I am under no illusions that these plants have an infinite lifetime. I know enough about embrittlement and displacement damage to know that you aren't going to run a decent-sized LWR, even with a low-leakage core, much beyond about 80 years. So if you can keep the failure rate low enough that you can manage that kind of timeline with essentially zero chance of a catastrophic failure, you're probably in a good position to make it.
BTW, one of my jobs as a consultant was to design a nuclear plant with essentially all replaceable components, including the pressure vessel. Looked great on paper. I tried to sell it to Circle Bar W but they didn't go for it, they were into IRIS at the time. Maybe I should dig those plans up...