I don't think payload volume is the driving factor, I think the question will be what yield per pound can they bank on?
The UK-Australia market is fairly unusual in lots of ways. One particular feature is when travelling such a long way it is expensive for passengers (and shippers) which means many people become unusually costs sensitive (even if they're not normally). So I suspect only a relatively small proportion of the market would pay a premium for non stop flights. Project Sunrise is about chasing the top end of yield.
There's a lot of history to QF's needs here. In short both BA and QF got badly hit by ME3 competition back in 747-400 days, because the ME3 (mainly Emirates) undercut on cost which is important for such a long trip, because 'cheap' is still 'a lot' of money to pay. BA and QF took different approaches of which BA's worked and QF's didn't.
BA went with a 300 seater premium heavy 777 solution and made money; QF went with the volume driven A380 and lost money for nearly a decade on LHR - which is hard to do (you can Google search these statements to check they're accurate, which they are). So when QF said they wanted a 300 seat LHR solution, but with the unique selling point of a non stop flight, it's not too hard to see where half their inspiration came from. The vast majority of people who fly between the UK and Australia make connecting flights via hubs because those 'connections' are usually cheaper, BA & QF's direct one stop flights are only a very small part of the market.
Project Sunrise is about chasing that top end market by offering a 'unique' quality point, a non stop flight. That cuts that BA/QF subgroup down even further. If QF think 300 seats works, but neither manufacturer can deliver that, the question becomes 'how much' will 'enough' people spend and can costs be kept low enough to make it pay.
As a 777 fan, unfortunately that in my view favours the A350 for three reasons:
1. The 777's strength is extra payload, which isn't QF's key driver. That extra payload comes at a cost in terms of crew/navigation etc etc and the market might not be big enough to sell that extra payload profitably.
2. The 777 costs more to buy (apparently, but who really knows what the deal might be?)
3. The A350-1000 has 3-4 times more aircraft on order, it is alive, well and performance (all be it not of this variant) can be judged. Not many people want the 777-8 with just 53 on order (of which the EY proportion looks unlikely to fly). Unfortunately not many people would be shocked if the 777-8 never flew or ended up being the unwanted A340-500 of Boeing's programme.
Each one of those points creates extra risk. QF would be wise to de-risk Project Sunrise, which is already pretty risky, as far as they can.
4. QF wants multifunctionality out of their fleet, so it needs to also be economical on shorter routes. Here, again, the 350 would win out.
As for the 'losing money for a decade' I did a Google search and was unable to find indication of this. Can you provide a link? Qantas masterfully aligned themselves with EK, which as far as I know helped to control capacity on the route. If the A380 was the cause of a lack of profit on the LHR route, would they not have just operated something else? They still had plenty of 747s at the time, and could have flipped the route to a 787 stopping in SIN if the aircraft was the cause. Something doesn't quite seem right there...
Also, I don't actually think that people on this route ARE that money sensitive. There are no LCCs or ULCCs operating the route (unless you switch to Jetstar or Scoot in SIN, which is quite rare). High costs are expected by even the economy flier. Most Australians don't expect to make it to Europe for much less than $2,000 AUD. Business Class is almost always well north of $6000 AUD, even for the less competitive airlines (Malaysia, Garuda, etc.). As well, Australians are routinely more aware of airlines they like and stopover airports they like (SIN, HKG, not so much for DXB, AUH, etc.), so they'll be more selective if the price only varies by a certain amount. I would say this is a much less price sensitive market than many others, especially TATL.
Refer to the following regarding QF profits or lack of them on London, since PER-LHR started LHR has returned to profitability for the first time since 2010https://www.airlineratings.com/news/qan ... nish-year/
As to why they didn't change it to a 747 or 787, the 787 has only been in the fleet for just over 18 months and the 747 while offering about 120 seats less than the A388, fuel costs would have still been high. On top all of this Australia to Europe has a lot of competition on it, when you break it down QF competes with BA, EK, QR, EY, AI, UL, SQ, MH, TG, MK, SA, CX, BI, GA, VN, PR, CI, BR, KE, OZ, JL, NH, MU, CZ, CA, HU, all of which fly to an airport in LON. Operating a non stop flight, as PER-LHR has demonstrated creates its own demand as it eliminates the stopover. Sure passengers pay extra for premium seating but economy pricing has not shown much difference.
As pointed above pricing between Australia and Europe is not really that price sensitive, most would budget around $2000 for economy and $6000 for business. On PER-LHR with QF you would pay at east $10000 return for business and have seen it up over $15000, economy has been around $1800-$2200 but it does go higher during peak periods. Even with the higher prices QF have indicated they could go double daily on PER-LHR if they had the frames which they don't at this point in time and by the time SYD-LHR and MEL-LHR are launched PER-LHR is likely to see the 777X or A350ULR as well. Also I would add (based on the daily numbers I see working at PER airport) the trend over the past year has seen passenger movements swing away from a transit in the Middle East and back towards the likes of SIN, HKG, BKK and KUL, I would go as far to say EK has lost their way and QR is now the dominant ME carrier out of PER. Whether a similar thing happens on SYD and MEL to LHR who knows.
Another thing that favors the non-stops is with us here in Australia being isolated from the rest of the world travelling to the US and Europe takes a good day of travel, if that can be minimised by cutting out a stopover and saving 2 or more hours there will be some that will appeal to that. There are approximately 13 million that are a QFF in Australia and to fill non-stop services on PER-LHR, SYD-LHR and MEL-LHR for a year would be just over 300,000 a year each way so there is definitely a market there for ULR non-stops. When PER-LHR started it was reported that LHR accounts for 30% of all traffic from PER to Europe, I suspect it is even higher from MEL and SYD