If I recall correctly, Valve used the release of HL2, LFD, and TF2 to leverage the distribution of Steam. This gave them a broad install base. Naturally, they had a way into homes and decided to use it. It's a tool that serves a function that most users enjoy. Once that usefulness wanes or a adverse effect of using it appears then it to will slowly pass. Think of AOL. Adapt or die is the business model that dominates our economy.
I find it funny that many people demonize successful models that provide a genuine service, many times because individuals feel they have gotten too large, or too successful.
As to the comment about having to buy multiple games for your household: Anytime I've had an online game I needed a second copy to be online at the same time on that game(Doom and Quake the exceptions). I recall any thing from ID Quake II range and on used punkbuster and did not allow same CDs keys at the same time. I had 2 soldier of fortune games for my nephew to join me online. Single player non-connected games were a different story.
It's been a long trip these last 30+ years. From 3-4 computers per community to 3-4 computers per home.
I believe that Valve/Steam will stay popular as long as they continue to innovate and provide services that individuals find worthwhile.