To be fair, Cocoon was effectively destroyed, however not to the extent that the people of Cocoon were destroyed - which is what the Cocoon fal'Cie were wanting, and which is what the l'Cie were trying to stop happening. Realistically I imagine that a large number of people would have died regardless when the gravity failed, however at the same time it should be noted that there were a large number of people who, under military "request", were evacuated. I would not be surprised if this would be at least 50% of the population going by the relatively low population (it's something like 10 million, which is more like London than Britain, if I recall correctly) and the large number of airships within Cocoon, and finally the fact that the amount of time between the order and the demise of Orphan would be fairly extensive.