Turtle Rock Studios made the in-house mod AND the final L4D game. Valve just happened to like the game so much that they bought the TRS crew and the L4D IP. After TRS (Or Valve south if you will) finished the game, a large part of the crew wanted to split from Valve again and Valve was kind enough to just give them back their old name, but not the Left 4 Dead IP.
I don't know a ton about the development of the Portal games or Dota for that matter, but if TF2 count as something Valve did themselves, shouldn't L4D2 as well? Aside from some DLC, TRS didn't get to touch that game. Not even the TRS people who stayed with Valve after L4D1 were allowed to work on it.
And although I don't know much about the development of the Portal games, I'm pretty sure they had a lot more than just the native team working on the second game. I think the point I'm getting at is that I think it's generally unfair to say that Valve hasn't been doing anything since TF2, and/or that they're no longer relevant, but that's just my humble opinion.