An example of a flat structure failure is the complete lack of a new episode of Half Life.
I don't buy your reasoning either. Right business move because DotA 2 and their Hats! are a cash cow business, sure.
They don't want to make Half-Life 3, the expansion to Half-Life 2. They want to make something new. They created something new with Portal, which required them to hone their ability to introduce players to an entirely new game mechanic, but now they have to wait for VR to put it all together.