They don't care how many people will buy HL3 at launch, their new platform is much more important for them. Steam was requirement for Half Life 2 and back in the days, Steam was godawful. Ugly, buggy, slow piece of shit client. But players were forced to use it to be able to play Half Life 2.
I also believe there's at least 5 million GabeN worshipers who will buy everything Valve makes. The likes of cmp aren't their loyal customers.