It should be noted, that the golden days of the automotive industry weren't quashed by previously existing large companies coming in from the outside the way you fear, but competition was culled down by giants that arose from the field of startups. Much of the competition in the early automotive days were locamotive makers that tried to make the transition from railway to road, but they were too slow to adapt and were pushed back out. It's probably safe to assume that 3D Systems and Stratasys are going to remain the biggest names in the market for a long time. Companies like HP and Brother aren't going to be able to make the transition the way you think they will, but might instead buy up smaller 3D printing companies to use their brand and expertise in the market.