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.