You can still eliminate inefficient stop-and-go driving for some fraction of stops (ie not U-turns), and (most important) the time-wasting "last 100 feet" walk. Sure it's not as superior in rural areas vs suburban, but with low enough overhead you could still beat a traditional truck.

Naturally if the drone gear cost millions of dollars and took up half the truck, then I'd agree it wouldn't be worth it. In reality I don't think that's the trade.

UPS showed off an interesting "roof mounted" hardware approach (ignoring how they depict of the rest of the ops model). https://www.youtube.com/watch?v=TYBu2glKHTA