Crucially we're not even talking about R&D efforts, insurance, regulatory clearing costs, and probably the biggest cost, legal mishaps - and I mean very serious lawsuits - even toy-like DJI Phantom drones can fall/ crash and kill people.
I could see drone delivery to rural areas... but urban areas I don't think it's feasible at all, at least not for a long while.
Drones could be useful for a lot of things but if the problem is delivering heavy packages cheaply to places with road access, they are not a rational choice.
By "happening" you mean "not used for any real purpose besides research or demonstration anywhere in the world" presumably.
From a regulatory perspective, I think driver-less cars are making better progress than drones, because auto regulations are more relaxed than air.
[1]http://www.dailymail.co.uk/news/article-3006700/Driverless-c...
... Oh god I just linked the daily mail. Sorry I'm at work and picked the first link when I googled.
In general, people take much longer to adopt new technology than we would like to see.
edit: grammar
"Two people will be in the specially rigged Audi in case of emergencies"