You are asserting without evidence that Valve needs to take all that money. As if they would go broke if they only took a quarter of all the revenue on most PC games.

Valve makes ten billion dollars on Steam, every single year. Their margins are not slim. And being an established de-facto monopoly, people go there because that's where the products are, and products are there because that's where the people go. They could slash costs to nothing, do the bare minimum work going forward, and still rake in the money on sheer momentum, for years and years and years.

The only feature that really matters here is adoption. And that's not a feature you can design. Even Valve didn't rope people in with a convincing sales pitch. They forced Steam onto everyone who wanted to play Half-Life 2. If you didn't want to put up with an always-online DRM service aimed to take over PC gaming - you didn't get to play the most anticipated game of the year. Whatever benefits you ascribe to the service, whatever functionality you argue developers would otherwise budget for, the core was always 'accept this or pound sand.'