This is an awesome idea of course. It's a glimpse into the future. I remember tweeting with Chris Anderson of 3D Robotics about a year ago about autonomous package delivery from A > B. I'm sure Bezos and team started nerding out on this concept years ago. Eventually, "vehicles" the size of UPS trucks will be delivering packages autonomously all over the world. This is definitely a glimpse into that day. And it won't…
How do you figure? Do you realize how big the sky is? For every square foot of land there are tons of layers of airspace above it, and it's almost completely unused except for a few tall buildings and a relatively small number of flying vehicles (compared to what we manage to fit on the ground.)
Drones are fairly maneuverable and could be made with decent collision avoidance and sensors.