I think Marc is right that players won't go back to a game many months down the road if it is essentially the same experience with reused content as the previous one.
We'll see. I bet HL:Ep2 and Ep3 will sell quite nicely. And be as dismissive as you like about the "sycophants" who buy these products, they're buying them because they enjoy them.
But that being said, I think that's more because Valve are actually quite good at making fun games that follow on from one another. For a series such as Sin, I think you're probably right - people won't pay for 9 episodes of that tedium.
What Marc is saying is that because these episodic titles are competing against full length games for the consumer's attention, they don't stand a chance.
Again, Valve's sale figures would disagree. What is true is that poor episodic games will not succeed. But that's true of most things.