And when LAX-SYD goes 777, it will probably be possible to hot-swap in case of MX, rather than waiting 24 hours for a new aircraft.
How many times have you heard of that happening when SFO-SYD goes MX? Maybe it has happened, but I struggle to think of a single time - despite SFO being a fairly major 747 base. They can pull it off on other routes (I had it happen on IAD-LHR last year, with a resulting ~3 hour delay), but crew hours seems to make it extremely difficult if not impossible for SFO-SYD unless they discover the MX very early.
I'm not saying your point isn't valid (it is!), but the SYD flights are a bad example.