Except that’s not what EC.261 says. It says that passengers should be compensated unless the delay is extraordinary, which is then helpfully not defined, and unless the delay would still have occurred if all reasonable measures were taken, which are helpfully not listed. The courts have seemingly agreed that adverse weather conditions are extraordinary, which is the sensible result but stretches the definition of “extraordinary” to its breaking point. The question here is whether or not the delay or cancelation of a return flight due to an out-of-position crew or aircraft is due compensation. Thus, an assertion that it is not reasonable to expect an airline to keep an infinite supply of spare planes and crews at every outstation is, in fact, quite germane — and I hope it’s how the Swiss regulator has ruled, because it’s not the least bit clear to me that the ECJ was able to make that same deduction. In fact, based upon ECJ cases, it appears that OP would have a case; under the UK standard, they would not; and under Swiss rules, I didn’t know the answer, which is why I asked.

None of this discusses whether or not the rule should exist, merely what it actually is.