I think simple math would help figure this one out.
From what I understand, Blizzard was sitting on Diablo 3 for a very long time.
Though I did come to a conclusion about Valve. The reason why they're holding off on Half life 3/episode 3 is simple; they know it's not gonna be that good and live up to the hype, but it's hard to tell your ravenous fan base that they really shouldn't be looking forward to it.
Half life 2 and all the episodes were good games... at the time. But the style of gameplay and traveling between point to point has been done time and time again, but much better. The only thing it really has going for it is atmosphere and character development.
But then again, Valve can redefine a series when it wants. Just look at Portal 2. Arguably better than the first one.
Though with a simple puzzle game like Portal, fixing up an extremely basic blank slate couldn't have been tremendously challenging. A game like Half life which has most likely reached it's peak will only go downhill.
But back on topic, I'm not much of a Blizzard kind of guy. World of Warcraft just seems intimidating to get into, Starcraft is very much for a select group of people (RTS's are not very healthy for a guy with limited amount of game time), and Diablo doesn't even sound fun, just addicting.