Falcon- Its not necessarily about pure economics.
For example, When the BAe 146 was first designed, there wasn't an engine powerful enough for the size to fit on the wing, so they had to go for a four engine design.
Along the same line- if you are going to design an aircraft of around 600Tonnes Max all up weight, then you aint going to get it into the air with a twin.
Why did Boeing develop the 747-8 if they thought it was dead?
(and I see the Airforce 1 contract is for a quad)
