I understand there's some kind of an argument going on here... but if I can just cut in for a second...
Okay, let us theorise for a moment that Half-Life 3, or Episode 3, has been released. The fanbase goes nuts for it, plays it, loves it, it becomes a historical shining example of FPS gaming, etc etc etc.
And then what is Valve left with? Half-Life has been their flagship for so long now, and even with all the side projects they have, when the series ends it's going to leave a bit of a void. What's going to fill it? Portal? I highly doubt it. They need to phase out Half-Life slowly, introducing us to new ideas and games that will take its place once it's gone. I don't see Valve as the types to milk a series beyond reason, so after Episode 3, they're going to need to put the series more or less to bed, and find something else to fill the spotlight.
Hence, my theory: The reason they are delaying HL3 is in order to work on other projects to satisfy their fans when it's gone. Perhaps we'll see a game based on the Seven Hour War, or perhaps they'll start a whole new franchise altogether, or simply focus on multiplayer gaming. It would be a shame if they didn't keep putting their design philosophy to good use, though.