The funny thing with Steam is the fact that most gamers aren't aware of what Steam can do to them, or what it really is. I talk to "young gamers" a lot (20-25 years), and they just fire up their CS or whatever, and play. I guess it's just us "old farts" complaining about the level of control and restriction that Steam (or online activation, like in BioShock) can put on us. Young gamers perceive copy protection schemes, even restrictive ones, as "normal" IMHO. They don't care if they have to verify their copy over the net. They don't care about Valve keeping track of which game has been played by whom.
I believe that Valve will keep Steam running as long as they can afford to, even if it wouldn't create any money (which i doubt - they are making loads of money with ads alone, i believe). I mean, they have created the de-facto standard in PC online gaming, whether we like it, or not. The platform is very well done from a technical viewpoint, and it works quite well most of the time. If they can keep it up long enough, there will be no real competitors. The only competition i can see would be MS with their XBL extended to PC's, but it seems that will take a while to become reality.
I don't like Steam very much, but i think Valve's business decision to create something like that platform based on the success of CS was genius. They saw the signs (Online gaming getting more and more important), and they acted accordingly. MS did the same with XBox Live, and that system is even more genius, with all the bells and whistles like achievements, system-wide score lists, and so on. It's fascinating how online gaming has evolved since the Doom/Quake days.