Why Qantas can't just switch Melbourne twice daily A330 to once daily A380?

In the ongoing debate of where Qantas is likely to schedule the three returning A380s over the coming year, several followers on social media have proposed the idea that Qantas could consolidate their two daily Melbourne-Singapore A330 flights into a single A380 operated flight.

From a capacity perspective this makes sense as each A380 carries 485 passengers whereas each A330-200 carries 251 passengers in Qantas’s international configuration. Consolidating two A330-200s into one A380 flight results is a negligible difference of 17 seats per day, and even on days where one flight is operated by the larger A330-300 it only amounts to a loss of 63 seats.

Consolidating the two A330s flights into one A380 flight generates a small capacity loss but it actually improves the premium mix as the A380 offers a first and premium economy product that the A330s don’t, while also providing more business class seats.

As others noted, this would be useful to provide a premium economy (and first) product onwards to London. This seems like a fairly compelling proposition, so what’s the problem?