Pretty sure I've never bought a game for Steam from anywhere BUT Steam. If I got a Steam key from somewhere other than Steam it was something like Humble Bundle and was basically free, so they're still screwed.Sure if you ignore Valve giving devs Steam keys which they can sell and keep 100% of the revenue.
I cannot see how that makes a big difference in the scheme of things.
Side note: Epic also waives Unreal engine fee for games on the Epic store.