Having had a recent chat with a former QF manager who claims involvement in the original 777/380 review this was what he said
- Decision at the time was one or the other - Qantas could not add another type (think Ansett)
- Qantas routes are very different to other airlines - key international requirement is ultra-long haul flights into slot restricted and time restricted airports (ie LHR,LAX but SYD restrictions play a factor) - means there are very limited flight windows that work
- On these routes - bigger is better so long as you can fill the plane, hence the 380 as Qantas rightly confident they could fill the plane.
- 787-8/9 fills a different mission on lower capacity routes or one where scheduling will enable multiple daily flights (eg into Asia)
Interestingly said he could see a role for a future 777NG if cockpit commonality with the 787 for flights needing more capacity, but not the 773 as for an airline that needs to keep its fleet (due to Aussie tax laws) for as long as QF that would be stupidity