There tends to be two things that can potentially fudge up either of those ideas, namely episodic content and planning trilogies before you even make the first game.
The problem with episodic content is what we're seeing with the Half-Life 2 episodes. The idea I guess is that people would rather have three small cheaper chunks over the course of a year and a half or two years rather than a brand new giant game every two or three years. The problem is that when you force yourself into a schedule like that you end up with delays, and that causes part of your "episodic advantage" to fizzle out.
The second is a similar problem that I like to call the Xeno Problemation....you see, with Xenosaga Monolith Soft planned to make six games over the course of however long time it took to make said games, the saga would involve a sweeping epic overarching plotline that would largely be divided between the first 3 and last 3 episodes( I think). So Xenosaga came out, it was awesome, got generally great reviews and sold well, but apparently not quite well enough. Episode II was an attempt to bring more casual Rpg players into the mix that couldn't stand the long cutscenes of Episode I( even though that's the whole reason you play a Xeno game, THE PLOT!!!) and subsequently they kinda fudged the game up trying to streamline it and subsequently the game got worse reviews and sold much less. As a result, the six episodes got cut down to three and the makers were forced to cram more than they expected into Episode III with a lower budget as a result of Episode II's lesser success but luckily Episode III did much better critically and still sold mildly well, though still not as well as they likely hoped.
Basically, you can't just expect to make three great games in a row, you have to make a game, if the game sells well THEN make a sequel, you can't just assume you're gonna make three blockbusters in a row. And on that note I'm hoping Xeno Ep. III turns out better than Ep II, cause I'm planning on buying it soon.
