You are probably right
Full or near full A380 is the only way to make a profit for the airlines who purchase the plane. The concept of a mix of passengers and freight might be a feasible solution though.
The smart airlines will be able to predict based on past demand of flights just how many passengers they expect and base the cost per passanger/gallon on that and not a full plane. The airlines who think, "build it and they will come" just might be doomed.
The Concorde cost something like 157 gallons per passenger and we know how that helped to kill it.
As far as my flying preferences the smaller the plane the better. If I could fly in a twin turbo prop everywhere I would be happier than in a 747.