A sample of one isn't enough, and in any event their success is not reproducible. They had a major title in Half Life 2, convincing people to install Steam, but that was their last major title and it is not clear that they used this process at the time.
Going forward: Someone else created Counter Strike, someone else created DoTA, someone else created Portal, someone else created Team Fortress, someone else create Left 4 Dead. Valve took these projects on and polished them, but they didn't really create something new until Artifact - which failed in the market.
What you also don't see is all the unannounced projects that went nowhere, because that's what you get when people just get to play around on their own, indefinitely.
Sure, the original idea might've already been there, but they built the game all the same. Saying it "wasn't them" is misleading. They built it from scratch, with existing concepts from mods.
Maybe their ideas department isn't the greatest, but their execution on these games was very good in the long run. They popularized hats and make absurd amounts of money from it.
Let's not forget the Steam Deck. Early on still, but an excellent product in a whole different area.
Going forward: Someone else created Counter Strike, someone else created DoTA, someone else created Portal, someone else created Team Fortress, someone else create Left 4 Dead. Valve took these projects on and polished them, but they didn't really create something new until Artifact - which failed in the market.
What you also don't see is all the unannounced projects that went nowhere, because that's what you get when people just get to play around on their own, indefinitely.