I remember the first time I heard about using an active Internet connection as a copy protection system, probably ten years ago, it sounded like a good idea for about five seconds, until I remembered that Internet connection is not eternally reliable and companies can come and go. Hell, the ISP I have now is hardly more reliable than the satellite DSL I had in 2001, and I can't even remember what now defunct publisher I first heard the idea from.
I've been afraid of the economic shift in how the videogame business operates for years now. When exactly did spending money to buy a copy of a game no longer mean that you actually own that copy? Imagine if you couldn't watch a recorded movie because you didn't have a live cable connection. Hey, every time you're watching something they're not providing, they're losing ad-share, and how does your cable provider know it's not an illegal copy anyway?
It's hard to think that Steam will ever not exist - the game industry seems to get more structurally stabilized all the time, and there's barely even other services trying to compete with Steam, let alone any that could actually bury them. But it's still hardly an impossibility that at some point in the future, Steam Service won't be there anymore. Will every game that needs it just up and stop working?