Actually, Turtle Rock's L4D was super far along when it went to Valve. I think Valve only had about a year with the game. While I agree with you re: Narbacular Drop, Turtle Rock's L4D > Valve's L4D. Valve without Turtle Rock went off and made Left 4 Dead 2, which added one essential monster but lowered the overall quality of L4D2 by a significant margin.
I'd just like to point out that Turtle Rock without Valve went on to make Evolve, the game that died and was forgotten days after it's release. Meanwhile, L4D2 is still pulling 10,000 people a day
6 years after it's release. Do you think that's an accident?
I'm always a bit baffled when people propagate this notion that 'Valve just hired creators and released their ideas'. It just completely ignores the amount of production work and thought that goes into anything they release. Those creators are given as much time and resources as they need to make a game as perfect as they can. Almost no one in the industry gets that opportunity. People should seriously find a Valve title with commentary mode and just listen to how much they think about every tiny detail in their games.
And then there is community building. No one does it as well as they do.
TF2 released 8 years ago. It has gotten hundreds of patches and content updates over the years. That one game has gotten more content updates than studios put out in their entire existence.
CSGO was a low quality disaster of a game when it released in 2012. A $15 title that most studios would have written off and moved on. Valve on the other hand actually started working on it and providing content and updates that the community wanted. The result? CSGO had 1 million monthly active users in December 2013. It has 9 million unique monthly users in December 2015. It has reclaimed it's position as the de-facto e-sport shooter. Has anyone ever seen a game turn around like CSGO? Was that an accident?
Dota 2 is also sitting on 11 million active players per month and growing. In an industry which manages to disgust you with new monetization schemes on a daily basis, they have set the standard for what a fair free to play model should look like. They have staunchly refused to ever sell anything outside cosmetics in their games. And they have grown Dota 2 into possibly the most lucrative e-sport around. Was that an accident?
On top of all of this they have built a system where talented creators can make content for their games. That content is then put in front of the community who get to decide what they want in their game. And then the creators of the chosen content are paid for their work. Valve have given out millions of dollars to these creators. So the content creator gets paid, Valve gets paid and the community gets entirely optional content that they chose into the game. Everybody wins. How does that $50, 4 map pack season pass look in comparison to this?
A company like Valve that bucks the industry wide trend of releasing annual or bi-annual sequels and instead chooses to support their existing games and communities for years and then people give them shit for 'not making games'. Okay then.