I don't think anyone really planned to have 600 people in it flying from UK-Singapore.
Even 555 would mean a high economy load and small F/J section.
If you look at the floor plan of the Qantas A380, there is a move for premium economy upstairs at the back.
The issue (and strategic intent of the aircraft) was to put more passengers on the ground in the same space [runway concrete occupancy time] more efficiently, and yes more luxuriously, than a 747 (including a 747-800).
Airbus went to great lengths to have the aircraft certified for 800+, even 'hiring' ex olympic athletes to be able to achieve the evacuation procedure standards of 90seconds from any seat.
The tightest constraint in aviation growth, particularly US and Europe, is runway availability! The A380 initially with a separation distance of 10NM [a 250% separation penalty] and now a 6NM [50% penalty] over the 747's 4NM spacing is a big issue. There are also major inflight separation spacing questions as well [horizontal and vertical].
With the model for airport slot pricing looking at changing away from one price fits all aircraft no matter the size [airports make money by passengers in the retail stores and carparks] then this is a major issue.
If the profitability of the flight for the airline is destroyed by the fact it takes up 2 slots [and pays twice as much, particularly in peak times], then 50 - 100 extra seats clearly makes the difference. On current measures, LHR is expected to see a -ve operational / throughput impact from the A380. Add extra taxi time / runway incursion issues- it becomes an interesting dynamic.
Customer market stratification / mix then receives a reduced priority.
This 'spacing' issue has a long way to run before being resolved. At the moment Airbus is still a long way from break even with the orders for the aircraft, and while there seem to be good fuel savings - potentially enhanced by the powerplant choice, whether you are able to economically land the aircraft (if at all) at the preferred destination is a real concern for any airline.
In the interim, it is up to the PR people to sell the benefits of more leg room / passenger mix, while the current economic climate does not help them attract full fare paying F/J passengers.
Still, with many more orders on the books for the 787 than the A350, I would expect the lead to market that Boeing have will result in similar delivery times should a customer sign up today (for A350s or 787s) - thus reducing Boeing's first to market advantage.
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