Well, this is the problem with popular series that are designed in an episodic format, despite the fire initially, there is no guarantee that any creative team is going to be able to keep that fire stoked all the way until the end, and if the team can't work without that fire? well the fans who committed are left wanting.
To be kind of honest, with all of the conjecture about G-man and a lot of the other wierdness in the series, I kind of suspect that Valve was making stuff up as they went along, and really didn't have an overall plan in mind for a lot of it. I've long suspected the delay on episode 3 is because... despite all claims... they really have no idea on how to finish this up in a way that will satisfy all of the fans.
To be honest these are the kinds of problems that happen in creative industries, and yet are never acknowleged. You don't see a company say "well, we don't care about the creative property that made us anymore, and can't get motivated" or "we kind of wrote ourselves into a corner and can't finish things in any way that will satisfy us", there might be an exception somewhere but it pretty much doesn't happen.
On a lot of levels I'm sort of glad I'm not a shooter fan who got invested in Half-Life... that lack of investment let's me voice concerns that I think are in everyone's minds at this point but they really don't want to contemplate.
Hopefully (for those who play the series) Gabe will shut me down (lol) by immediatly announcing a solid release date for "Episode 3" but I kind of doubt it.
Also on a final note, I think Zombie-Mania still isn't dead yet despite all logic governing trends. Like it or not, "Left 4 Dead 3" would probably be a wiser business move from a pure money making perspective than releasing "Episode 3". Especially seeing as I don't think anyone has yet to actually make the definitive Zombie Apocolypse game... but Valve seems to hav come the closes which is why so many people seem to be trying to copy their formula. Dead Island owed a lot to how Left 4 Dead was set up.
