At this point in time, 3D printers will have almost no impact on mass manufacturing.
The injection molding process produces almost no waste, and its extremely fast compared to a 3d printer. Also uses far less energy. No matter how good 3d printers get, it probalby will never replace injection molding for simple items.
For prototyping, 3d printers rule. Even tho they have been used for this purpose for about 25 years now, the price coming down will make them much more prolific for this purpose.
For customization, 3d printers will rule. But this is a niche industry, not mass market.
3d printers hold a lot of promise, but they are still a long way off for being a game changer for the common person.