Well this is an interesting question, and there's a good answer: because if you make the drone dome dense enough, the drones can be so slow they don't move. So,

Well this is an interesting question, and there's a good answer: because if you make the drone dome dense enough, the drones can be so slow they don't move. So, the less dense you make it, the more they have to move, so there is a nice optimization problem with the lower bound of density given by the speed of the drone, and the upper bound based on cost and complexity. It also matters how big the dome is - you want a big one, but if its too big density goes down again.