Valve's never milked anything before.
Well, except Gabe Newell, but that's a legitimate health thing, his teats could explode if they don't keep that up.
And L4D2. But, besides that... everything they've made has been solid gold and more than value for your money. With Portal they just basically copied and expanded on a concept another company had done but it's plainly obvious to anyone with an imagination that there's tons more potential. Try downloading some of the unofficial level packs!
Maybe more importantly, EP2 strongly hinted towards a Portal crossover, and some of the viral stuff has seemed to be that direction as well. Apparently there's a voiceover by Alyx somewhere in all that mess. So Portal 2 could be seen as Episode 3 if that proves to be the case, which means... HALF LIFE 3!
I'm stretching a bit but damn I'd go for cryo-freezing if it meant I could emerge in a world with HL3 in stores.