Imagine if they put a small warehouse in a big city, filled with their most commonly sold items in that city. (For now we'll pretend the warehouse/dronepad is on the roof of their Seattle headquarters, right in the center of Seattle)
I can SEE Amazon's buildings from my apartment right now. It's probably a mile or two as the crow (or drone) flies. That seems well within the range of what Bezos talked about for Prime Air. I also have a rooftop deck on my building that only residents can access. It's pretty big and would be a perfect place for a drone landing pad.
There are probably a hundred similar apartment buildings all within a 10 mile radius of Amazon. They would all be able to easily handle drone deliveries from this small, centralized, micro-warehouse.
Can commercial drones handle straight line flying with building sized object avoidance over a space of a couple miles? I think so.
Will they "in a few years" just like Bezos said? Absolutely.
Just because it won't work in rural south dakota or the suburbs of Detroit Michigan, doesn't mean that it won't work somewhere.
Like the author said, Amazon trialed fresh for FIVE years before taking it out of Seattle. What's to say they won't do the same thing with Prime Air?
(Just in case anyone is wondering, I don't work for Amazon)