If we had 2 fleets, we then have double the chance of a fleet grounding. A grounding that takes down half the fleet is no less crippling than a grounding that takes down the entire fleet. That is also a very low probability event. The added complexity however would be evident from day 1, with an absolute certain probability of occurring. So the added benefit of mitigating a low probability event does not outweigh the added cost/complexity to our model.