I would think that once you're sending out trucks anyway, there isn't much even potential win in layering a complex air delivery system on top of that. In addition, you're adding another source of latency and, to the degree that drone delivery makes any sense at all, I expect it would be used for very time sensitive material.
Maybe not trucks specifically, but I would imagine that having the notion of mobile or temporary drone launch points would help a lot during busy times - e.g. the week before christmas.
I watched all the major carriers sending out multiple delivery dispatches a day through my neighborhood. To the point that the bottleneck appeared to be concurrency issues on driver time (as I tracked a few packages through the various systems). For my family, many of the packages fell within reasonable drone parameters and a lot of them had a significant pause time at local delivery base (e.g. UPS truck loading center or post ofice). I imagine a steady stream of drones from a temporary drone base would help lighten this load. Imagine sending a semi-truck that operates as a drone base to a big parking lot, appropriately located to be in a delivery dense area. A handful of employees does piloting and battery swapping and drone loading, while a delivery truck brings packages to the temporary hub. When the busy and/or overload time is over, the truck can go back home, or on to the next place expecting an overload. It wouldn't take a lot of saved overtime and seasonal labor costs to make this a win.
edit: I would also imagine that a lot of driver time during christmas anyway is wasted on traffic issues, part of which are compounded by them! Drones wouldn't have this so much.
But drones are much more range- and speed-limited than trucks are. It might very well make sense to drive a truck from New Jersey to Manhattan, or from Irving to Plano, and then let the drones spread out.
We know that much of a delivery network's cost is the last mile, but I'd be interested in the costs of the last 100 yards. Suddenly, instead of one driver/one truck per neighborhood per day it's one stop per neighborhood per day. They can stop once or twice in the neighborhood, load drones with packages and then drones do all the deliveries. If you even have a separate "loader" in the back, you can have the driver jump out and make the few targeted heavy package (20+lbs) deliveries.
