It's a similar situation to what happened to the fallout series. Bethesda took a stab at it, but, if you loved fallout 1 and 2, nothing they made really felt like fallout, even if they were good games.
Not quite on the level of duke nukem forever, mainly because apogee was mismanaged at it's very core, and valve, well, hard to say that it's not managed well.
The thing is, making a good game is not really necessary here. They could make a mediocre halflife3, get slaughtered in the reviews, and still make a ton of profit, Gaben just won't do it because he doesn't want to go out like that. Understandable, but, it's mostly his fault that he's in this situation. If they released 3 a couple years after the episodes, they could have continued that incremental release pattern more or less indefinitely, it's just that at the time they were focused on other things (that turned out to be more profitable) and by the time they got back to making games again, it was too late, too much hype to live up to, so on the shelf it goes.
