Steam machines are simply ways to easily access steam, a cloud service for video games. Cloud computing is the future of computing in general, so it seems somewhat naive to think that cloud computing and third party development won't be future of video gaming. Assuming this is to be true, steam will be successful.
The Steam model of downloading software (generally games) online rather than buying hard copy instances of software programs will become more and more popular due to two pressures: 1. The cost of manufacturing. 2. The customers growing unfamiliarity with the actual processes involved with making software run. The cost reason is simple: Manufacturing x number of CDs and then shipping and storing them cost money. The unfamiliarity reason is almost as simple: as the ubiquity of these machine becomes more and more prevalent, the understanding of them will diminish resulting in the mindset of "Machine, bring me my game." The steam model will bring people their game.