You underestimate the significance of onslaught mode. It is radically different from anything in UT2003, and the fact that it was and still is by far the most popular of UT2004's modes played online demonstrates that. It's no mere team deathmatch addition to deathmatch or even something like instagib.
Episodic games are all about reusing game content to deliver a shorter game experience for a lower price. Game developers like it because it allows them to have more products to sell to the consumer over the same period of time than if they were developing a traditional full length title. 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. At the episodic games' current price point I agree with him because the consumer can find plenty of full length games selling at the same or lower ptices than the episodic ones.
I'm not sure I agree with the reusing game content comment, but we'll have to see how different it is when HL2:EP2 and Sin episodes 2 come out. HL2:EP2 is supposed
to be pretty different from HL2 and HL2:EP1 (since EP1 is pretty similar to Half Life 2). Sin episodes, we'll see. Then again, Sin episodes had the tagline "and everything... EXPLODES!" so what can you expect there.