...So basically it's the story that's stopping them?
Yeah, looks like it.
I mean they have to be really careful here. I mean look at Mass Effect 3. People hated the ending so much that they've even started trying to get the fucking FCC on the case. For a game's ending. Pretty much everything else with the game was awesome. But about 5 minutes of the game caused immense hatred towards it. Valve can't do that same.
Half Life is incredibly story driven and if they did anything to ruin the environment or characters, then Valve would be eaten alive. I mean there's so much subtle-yet-incredible things about the series that making the games takes as long as they do. Portal is rather similar. There's so many subtleties that it took them like what, 4 years to get around to a sequel? The game itself isn't even that long. The puzzle designs are really well done but I doubt it took them 4 years to do those puzzles alone. And the engine itself is already built for all these games and it's so easily adjustable and changeable that most any development time in that field is probably incredibly short.
And there's some sort of Shingeru Miyamoto quote, like "A delayed game can still be good, a bad game is bad forever" or something along those lines. It's Valve's philosophy.