As you've noted, these situations are not easy. If DL40 is canceled, DL would lean on VA and their (usually) 3 flights daily to MEL, BNE, & SYD. The VA option is especially nice because they can also feed onward traffic inside Australia and NZ. That's 3 VA 777 that can absorb some of the blow. Passengers making international connections at SYD (would mostly be to NZ, but could be to DPS, pacific islands, etc) would probably be routed on non-partner carriers on a more direct routing to their destination. Passengers going to DRW or PER might be routed through Asia. Passengers coming from the east-coast USA, if the cancellation is known far enough in advance, could also route through Asia or via other airline's flights from Texas.

The hardest part really is J pax... it tends to run higher capacity on this route than Y (IME). Paid J pax would likely be routed on any carrier with space.

It's kind of amazing how well the airlines can handle a single cancellation like this honestly. It's when weather, airport closures, etc cause multiple cancellations that things really melt down.