That is ordinarily true, but when was the last time you saw a sale fare on PER-LHR? Every time I've checked, there is a serious premium in all classes for this route. Qantas is on the public record that it is their single most profitable route, and performing well ahead of expectations. Unless you think they are blatantly lying (which would likely be a criminal offence, given it would be market-sensitive information), in this case I would say it is fairly safe to say that this route has excellent RASK and profitability.
If that is the case re fare picing then this is a great outcome for Qantas (I have not done any fare comparisons). The other part to it is also that people will try it to see if it is a better way to travel over current routings, so I think there would be a percetange of RPK's that simply wanted to get to London faster which is really a misnomer at the end of the day - it is still one stop to London for the majority (only PER consumers benefiting) - it just happens to be in Australia. End of the day - some will love it and some will loath it. For me - I could not stand sitting onboard for that long. I've done DFW twice - once in J and once in P - and I still hated it. I don't know how others can do it in Y. I just could not do that long. But that is me.
We'll see how it continues over the coming cycles.