tdracer;
This is not strictly true, the Boeing marketing people made lots of commitments to airlines that the 787 would deliver a particular seat mile cost at 8-abreast in Y. Airlines bought the plane and factored it into their business models on this basis. However as it turned out after the development fiascos those commitments could only be met if an extra seat per row was added.
If all airlines cared about was the absolute lowest cost in Y then there would be hundreds of A350s with 3-4-3 down the back, whereas in actual fact there are only a couple (with Air Caraibes).