Reading through some of the replies, I wonder at some of the negativity towards Valve here:
First off, Portal was great. That's all. Oh and the sequel was an improvement in every way. What more can anybody ask for?
Second, Valve has been busy lately. You know, Steam, TF2, Dota2. You know, great sales and arguably the best digital only game distribution service anywhere. And free AAA games with business models that don't appear to amount to pay-to-win, but may well be carrying their weight, money-wise, from Valve's POV. Valve is nearly every bit the gaming company that Blizzard is, but without the evil.
Third, if you look at what Steam managed to do with Source over the last decade, why in the world wouldn't they come out with their new engine just in time to coincide with the next consoles. This way they make sure that they have something really next gen to build games upon for the next decade. And that HL3 should be the game to usher in the new engine shouldn't surprise anybody, given their history.
That said, HL3 can't get here soon enough. I'm sure that it'll be a blast.