There's a very important detail that is missing here, namely that Steam has a clause where you cannot list your game elsewhere for less than you list it on Steam. Many indie games are published to Itch.io and Steam, but Valve forces them to list both at the same price, meaning thay since most gamers use Steam they will automatically choose the version that invisibly only gives the devs 70% of the revenue. Sure Valve does have lots of nice features, but those features could be duplicated over time by one of Valve's competitors. The problem is that to get a competing platform off of the ground, you need to entice people with lower prices since you can't compete on features. Valve uses its monopoly status to make this impossible.