koppelman:

That's not really the complete rationale. Both Boeing and Airbus aim to provide a complete range of aircraft for their customers to choose from, so they can scoop the entire business. The only significant difference is that Airbus will have one size larger than Boeing. The A340-600 is pretty comparable with the B777-300ER in terms of size (longer but thinner). It seems that Boeing will drop the 747 soon, sales have ben very slow for some years now.

There were exactly the same discussions in the 1960s when Boeing came out with the 747. What is surprising is how LONG it has taken for anybody to offer anything larger than that. You will notice the 747 is less used nowadays across the Atlantic due to the multiplicity of operators and gateways on both sides, but it is much more common across the Pacific, and for flights from Asia to Europe, where these considerations do not apply.

Boeing would love to be in all areas of the market too. Their real problem is that after a couple of decades of steadily increasing competition from Airbus they do not have the funds available to develop new competitive aircraft. All the recent Boeing derivatives (737-900, 757-300, 767-400, even the current 777-300) have all been commercial flops, which must come as a real surprise to them - Boeing traditionally have not had flops, which used to be left to the Europeans! They can probably afford the new 7E7 to replace the 757/767, but they must be asking themselves if this is going to be the real money-spinner they desperately need. Boeing cash flow at present comes mainly from the 737, a 1960s design outclassed technically by the A320 series. Their only way to compete is to charge less for it, which they do but this does not generate good R&D funds for the future.

The A380 brings further spectacular technical advances in the Airbus style, such as the composite aluminium/glass fibre material used for the aircraft structure. I think the real market for the A380 will be with a main deck cargo area like the 747 Combi, giving as many seats as a current 747 and about as much freight space as a 747 freighter. The economics of that will be unbeatable.