I don't agree that rebooking onto other flights/carriers was a reasonable option in this case. It's not one or two pax who have misconnected, it's an entire 787 load that would need to be reaccommodated, which is an incredible amount of work for whoever has to handle it. There are no other flights to Australia from DFW on any carrier, except for Qantas the next day, and no domestic flights out to other ports like LAX or SFO until much later in the morning. The international flights from those US west coast ports to Australia wouldn't leave until the next evening, a timing driven by SYD's curfew but tends to work out for other destinations too, due to connections on both ends.
Also, never mind that US domestic flights tend to operate at very high load factors these days, so getting an entire 787 load of people to LAX/SFO (even HNL?), wouldn't be easy, even if there were seats available onward to Australia. And most importantly, none of that gets the passengers to Australia any earlier than simply waiting until the plane in DFW was fixed and crew available again. So yes, it was way cheaper for Qantas to get the pax to wait until the same plane could be used again, but it was also the only reasonable option here if you actually look at the passenger point of view (many who have chosen DFW to avoid places like LAX to start with). I think you need to focus on why the process of getting accommodation, and the communication on the revised departure time, didn't work any better.