I can agree with this mostly but, I never saw Portal 2 as anything majorly different from Portal 1. It was more of the juicy same goodness, if not better because there was more of it. It looked better, the controls were better, the UI was better, etc etc. However I do think what SHOULD be focused on for sequels is a MAJOR engine change, otherwise in todays world, just update it with patches and DLC.
I say this because as Jim pointed out, games aren't films. I get the whole weaving the narrative in, but personally with things like Portal 2 and Bioshock, I think games have mastered the basic in storytelling for games. Or, ya know, we have that thing that everyone else did (start a level with a cutscene, play the level, end with a cutscene). I mean, yeah that's linear gameplay, sandbox would need more looking into, but still, we've pretty much got it in the bag.
Now my major point is with this is something like Fallout New Vegas. Ahem, new story, new character, new world, yes, thanks, appreciate it, but if it runs off the same engine, why not just release it as DLC for like £20-25 (considering a game here is usually £40). The story was fine in New Vegas, in fact I'd hazard to say I liked it more than Fallout 3, HOWEVER, it's the same old? Why did I need the other disk?
This way you just buy the usage off of one engine really and then update it with so many stories. You know who this would benefit greatly? Indie developers. Download a game using a major companies engine for like £15, I'd be on that like no tomorrow. And perhaps £5 would go to the engine license holders, I dunno, economics is a downfall to me, but to me, this seems like such a better model. I think it would be most beneficial to Valve, I mean I know they're leaving the episodic gaming format but still...this way developers only REALLY have to focus on the storytelling aspects compared to "Oh noes, his back needs more render power to make it glisten in water" for a sequel change...yay, just what I always wanted...