Do you really think any new technology will somehow overwhelm the economy of scale of printing thousands and thousands of copies of a product?
It doesn't have to.
To be successful, PoD has to beat the total cost of getting you a hardcopy the traditional way, through the whole distribution chain - that means printing, storage, and shipping, and the required work for tracking the hardcopy through its passage, combined.
It is still a tall order, but perhaps a touch more realistic.