Source 2 is what Valve have been waiting to finish before they bring out Half Life 3, Portal 3, L4D3 and TF3.
Yes Half Life 3, I said it. Half Life THREE.
What follows Source 2 is the '3' games. Afterall, you wouldn't continue your award winning franchise that everyone has been waiting for, on a aging engine, would you? And less face it, Source 1 is so behind modern engines, Valve really needs Source 2 finished to show HL3 in the best way possible.
They need their own games to use as tech demos to get other developers onto using Source 3 too. And with their console coming out at the end of the year they need first party games to help it sell. Not saying that those sequels will come out that quickly with the console but you need Source 2 out before the '3' games come out and with the Steam console out soon, it makes sense for Valve to bring out the '3' games sooner than later. So it's getting closer guys.