Last mile delivery will eventually be a huge market for UAVs. Think beyond just tacos -- think about anything that people would pay a premium for to have it delivered to them. Out of milk? No problem. Need your prescription filled? Delivered to your GPS coordinates. Condom delivery? FedEx?
The technology is there. The regulations need to catch up.
Oddly, the technical challenge of sending a ground robot through an unknown dynamic environment is even more difficult than a flying robot. There's very few obstacles in the sky. Another thing to consider is petty theft and vandalism. Maybe I'm jaded, but I think a ground robot would not last very long. With an aerial vehicle, packages can be dropped with parachutes. (though, I'm pretty sure some will be shot down before they're generally accepted)
The technology is there. The regulations need to catch up.