I always thought that in the case of QFd they would tug any available planes to a flight thats been cancelled so that that flight could still be replaced. Ie still fly.
Never realised that QFd would just cancel a flight and not tug in another plane.
Like all airlines, AustraliaPoochie, QF might very well like to be able to do that.
However, when almost all serviceable planes are in use - to make a generalisation for Australian domestic operations, most likely on Monday mornings, Friday noon to stumps and Sunday afternoons/ evenings, and during school holidays, Christmas and other public holidays - typically there will not be a plane available unless another flight on the same or another route is suddenly cancelled. And then it's not just one flight that becomes a no show - typically it is at the very least a return flight.
On top of that even at slacker times such as Saturday afternoon when QFd and its competitors may not have all the serviceable fleet in use, if a plane fails suddenly in say ASP, there is no guarantee that there will be a suitable QF plane (or indeed any QFd plane) to 'replace' the failed aircraft. And then there's how (if fully booked, or even 80 per cent full) a B717 cannot generally replace a B738, and then how flight and cabin crews may be qualified on one model of aircraft but not on another, and there are no spare crews in the location that the first plane failed...the list goes on and on....
Remember that like rail and bus surface modes, airlines schedule maintenance so fleet availability is rarely 100 per cent for any type of plane in an airline's fleet - and as you discuss, there are unscheduled failures to boot that while relatively rare as a percentage of sectors flown can and do occur. There must be tens of reasons why at the very least, if not in the hundreds if one was technically minded or qualified to fly and considered all the not quickly repairable items that could go wrong and were sufficiently serious to cancel a flight.
I doubt that any airline pilots or accompanying staff want a flight ever to be cancelled, but with safety paramount, unfortunately it must sometimes occur, even if passengers' plans are disrupted (and as a side issue, airline staff may on occasion be late home for that all important family birthday or other special event). Annoying, but there are sufficient occasional safety-related breaches or incidents without wilfully adding to them. Overall, our RPT (regular public transport) aviation sector in Australia has a very good (though not perfect) safety record.